diff --git a/lib/papi-dts/papi.d.ts b/lib/papi-dts/papi.d.ts
index 5271315e53..e32f449b35 100644
--- a/lib/papi-dts/papi.d.ts
+++ b/lib/papi-dts/papi.d.ts
@@ -2487,10 +2487,14 @@ declare module 'papi-shared-types' {
     /**
      * Set the value of the specified project setting on this project.
      *
+     * Note: `setSetting` must call `papi.projectSettings.isValid` before allowing the setting
+     * change.
+     *
      * @param key The string id of the project setting to change
      * @param newSetting The value that is to be set to the project setting.
      * @returns Information that papi uses to interpret whether to send out updates. Defaults to
      *   `true` (meaning send updates only for this data type).
+     * @throws If the setting validator failed.
      * @see {@link DataProviderUpdateInstructions} for more info on what to return
      */
     setSetting: <ProjectSettingName extends ProjectSettingNames>(
@@ -4709,6 +4713,24 @@ declare module 'renderer/hooks/papi-hooks/use-dialog-callback.hook' {
 }
 declare module 'shared/services/project-settings.service-model' {
   import { ProjectSettingNames, ProjectSettingTypes, ProjectTypes } from 'papi-shared-types';
+  import { UnsubscriberAsync } from 'platform-bible-utils';
+  /** Name prefix for registered commands that call project settings validators */
+  export const CATEGORY_EXTENSION_PROJECT_SETTING_VALIDATOR = 'extensionProjectSettingValidator';
+  export const projectSettingsServiceNetworkObjectName = 'ProjectSettingsService';
+  export const projectSettingsServiceObjectToProxy: Readonly<{
+    /**
+     *
+     * Registers a function that validates whether a new project setting value is allowed to be set.
+     *
+     * @param key The string id of the setting to validate
+     * @param validator Function to call to validate the new setting value
+     * @returns Unsubscriber that should be called whenever the providing extension is deactivated
+     */
+    registerValidator: <ProjectSettingName extends keyof ProjectSettingTypes>(
+      key: ProjectSettingName,
+      validator: ProjectSettingValidator<ProjectSettingName>,
+    ) => Promise<UnsubscriberAsync>;
+  }>;
   /**
    *
    * Provides utility functions that project storage interpreters should call when handling project
@@ -4730,11 +4752,11 @@ declare module 'shared/services/project-settings.service-model' {
      * @returns `true` if change is valid, `false` otherwise
      */
     isValid<ProjectSettingName extends ProjectSettingNames>(
+      key: ProjectSettingName,
       newValue: ProjectSettingTypes[ProjectSettingName],
       currentValue: ProjectSettingTypes[ProjectSettingName],
-      key: ProjectSettingName,
-      allChanges: SimultaneousProjectSettingsChanges,
       projectType: ProjectTypes,
+      allChanges?: SimultaneousProjectSettingsChanges,
     ): Promise<boolean>;
     /**
      * Gets default value for a project setting
@@ -4753,6 +4775,18 @@ declare module 'shared/services/project-settings.service-model' {
       key: ProjectSettingName,
       projectType: ProjectTypes,
     ): Promise<ProjectSettingTypes[ProjectSettingName]>;
+    /**
+     *
+     * Registers a function that validates whether a new project setting value is allowed to be set.
+     *
+     * @param key The string id of the setting to validate
+     * @param validator Function to call to validate the new setting value
+     * @returns Unsubscriber that should be called whenever the providing extension is deactivated
+     */
+    registerValidator<ProjectSettingName extends ProjectSettingNames>(
+      key: ProjectSettingName,
+      validatorCallback: ProjectSettingValidator<ProjectSettingName>,
+    ): Promise<UnsubscriberAsync>;
   }
   /**
    * All project settings changes being set in one batch
@@ -4768,7 +4802,20 @@ declare module 'shared/services/project-settings.service-model' {
       currentValue: ProjectSettingTypes[ProjectSettingName];
     };
   };
-  export const projectSettingsServiceNetworkObjectName = 'ProjectSettingsService';
+  /** Function that validates whether a new project setting value should be allowed to be set */
+  export type ProjectSettingValidator<ProjectSettingName extends ProjectSettingNames> = (
+    newValue: ProjectSettingTypes[ProjectSettingName],
+    currentValue: ProjectSettingTypes[ProjectSettingName],
+    allChanges: SimultaneousProjectSettingsChanges,
+    projectType: ProjectTypes,
+  ) => Promise<boolean>;
+  /**
+   * Validators for all project settings. Keys are setting keys, values are functions to validate new
+   * settings
+   */
+  export type AllProjectSettingsValidators = {
+    [ProjectSettingName in ProjectSettingNames]: ProjectSettingValidator<ProjectSettingName>;
+  };
 }
 declare module '@papi/core' {
   /** Exporting empty object so people don't have to put 'type' in their import statements */
diff --git a/src/declarations/papi-shared-types.ts b/src/declarations/papi-shared-types.ts
index c475b5b1a5..c50841fb8d 100644
--- a/src/declarations/papi-shared-types.ts
+++ b/src/declarations/papi-shared-types.ts
@@ -174,10 +174,14 @@ declare module 'papi-shared-types' {
     /**
      * Set the value of the specified project setting on this project.
      *
+     * Note: `setSetting` must call `papi.projectSettings.isValid` before allowing the setting
+     * change.
+     *
      * @param key The string id of the project setting to change
      * @param newSetting The value that is to be set to the project setting.
      * @returns Information that papi uses to interpret whether to send out updates. Defaults to
      *   `true` (meaning send updates only for this data type).
+     * @throws If the setting validator failed.
      * @see {@link DataProviderUpdateInstructions} for more info on what to return
      */
     setSetting: <ProjectSettingName extends ProjectSettingNames>(
diff --git a/src/main/data/core-project-settings-info.data.ts b/src/main/data/core-project-settings-info.data.ts
index 94843ebb45..f2fac0e858 100644
--- a/src/main/data/core-project-settings-info.data.ts
+++ b/src/main/data/core-project-settings-info.data.ts
@@ -1,4 +1,9 @@
+import {
+  AllProjectSettingsValidators,
+  ProjectSettingValidator,
+} from '@shared/services/project-settings.service-model';
 import { ProjectSettingNames, ProjectSettingTypes } from 'papi-shared-types';
+import { stringLength } from 'platform-bible-utils';
 
 /** Information about one project setting */
 type ProjectSettingInfo<ProjectSettingName extends ProjectSettingNames> = {
@@ -27,4 +32,27 @@ const coreProjectSettingsInfo: Partial<AllProjectSettingsInfo> = {
   'platformScripture.versification': { default: 4 },
 };
 
+// Based on https://github.com/paranext/paranext-core/blob/5c403e272b002ddd8970f735bc119f335c78c509/extensions/src/usfm-data-provider/index.d.ts#L401
+// Should be 123 characters long
+// todo Check that it only includes 1 or 0
+export const booksPresentSettingsValidator: ProjectSettingValidator<
+  'platformScripture.booksPresent'
+> = async (newValue: string): Promise<boolean> => {
+  return stringLength(newValue) === 123;
+};
+
+// Based on https://github.com/paranext/paranext-core/blob/5c403e272b002ddd8970f735bc119f335c78c509/extensions/src/usfm-data-provider/index.d.ts#L391
+// There are 7 options in the enum
+export const versificationSettingsValidator: ProjectSettingValidator<
+  'platformScripture.versification'
+> = async (newValue: number): Promise<boolean> => {
+  return newValue >= 0 && newValue <= 6;
+};
+
+/** Info about all settings built into core. Does not contain info for extensions' settings */
+export const coreProjectSettingsValidators: Partial<AllProjectSettingsValidators> = {
+  'platformScripture.booksPresent': booksPresentSettingsValidator,
+  'platformScripture.versification': versificationSettingsValidator,
+};
+
 export default coreProjectSettingsInfo;
diff --git a/src/main/services/project-settings.service-host.test.ts b/src/main/services/project-settings.service-host.test.ts
index 92fdbbf481..307a36ceb3 100644
--- a/src/main/services/project-settings.service-host.test.ts
+++ b/src/main/services/project-settings.service-host.test.ts
@@ -1,6 +1,14 @@
 import { testingProjectSettingsService } from '@main/services/project-settings.service-host';
+import { ProjectSettingValidator } from '@shared/services/project-settings.service-model';
 
+jest.mock('@shared/services/network.service', () => ({
+  ...jest.requireActual('@shared/services/network.service'),
+  registerRequestHandler: () => {
+    return {};
+  },
+}));
 jest.mock('@main/data/core-project-settings-info.data', () => ({
+  ...jest.requireActual('@main/data/core-project-settings-info.data'),
   __esModule: true,
   default: {
     'platform.fullName': { default: '%test_project_full_name_missing%' },
@@ -10,19 +18,34 @@ jest.mock('@main/data/core-project-settings-info.data', () => ({
     },
     // Not present! Should throw error 'platformScripture.versification': { default: 1629326 },
   },
+  coreProjectSettingsValidators: {
+    'platform.language': async (newValue: string): Promise<boolean> => {
+      return newValue === 'eng' || newValue === 'fre';
+    },
+  },
 }));
 
 describe('isValid', () => {
-  it('should return true always - TEMP. TODO: Fix when we implement validation #511', async () => {
+  it('should return true', async () => {
+    const projectSettingKey = 'platform.language';
     const isSettingChangeValid = await testingProjectSettingsService.isValid(
-      '',
-      '',
-      'platform.fullName',
-      {},
+      projectSettingKey,
+      'eng',
+      '%test_project_language_missing%',
       'ParatextStandard',
     );
     expect(isSettingChangeValid).toBe(true);
   });
+  it('should return false', async () => {
+    const projectSettingKey = 'platform.language';
+    const isSettingChangeValid = await testingProjectSettingsService.isValid(
+      projectSettingKey,
+      'ger',
+      '%test_project_language_missing%',
+      'ParatextStandard',
+    );
+    expect(isSettingChangeValid).toBe(false);
+  });
 });
 
 describe('getDefault', () => {
@@ -42,3 +65,17 @@ describe('getDefault', () => {
     ).rejects.toThrow(new RegExp(`default value for project setting ${projectSettingKey}`));
   });
 });
+
+describe('registerValidator', () => {
+  it('should resolve', async () => {
+    const projectSettingKey = 'platform.fullName';
+    const fullNameSettingsValidator: ProjectSettingValidator<
+      'platform.fullName'
+    > = async (): Promise<boolean> => {
+      return true;
+    };
+    await expect(
+      testingProjectSettingsService.registerValidator(projectSettingKey, fullNameSettingsValidator),
+    ).resolves.toStrictEqual({});
+  });
+});
diff --git a/src/main/services/project-settings.service-host.ts b/src/main/services/project-settings.service-host.ts
index 2a8348492f..bd7154ec23 100644
--- a/src/main/services/project-settings.service-host.ts
+++ b/src/main/services/project-settings.service-host.ts
@@ -1,12 +1,19 @@
+import * as networkService from '@shared/services/network.service';
 import coreProjectSettingsInfo, {
   AllProjectSettingsInfo,
+  coreProjectSettingsValidators,
 } from '@main/data/core-project-settings-info.data';
 import networkObjectService from '@shared/services/network-object.service';
 import {
+  CATEGORY_EXTENSION_PROJECT_SETTING_VALIDATOR,
   IProjectSettingsService,
+  SimultaneousProjectSettingsChanges,
   projectSettingsServiceNetworkObjectName,
+  projectSettingsServiceObjectToProxy,
 } from '@shared/services/project-settings.service-model';
+import { serializeRequestType } from '@shared/utils/util';
 import { ProjectSettingNames, ProjectSettingTypes, ProjectTypes } from 'papi-shared-types';
+import { includes } from 'platform-bible-utils';
 
 /**
  * Our internal list of project settings information for each setting. Theoretically this should not
@@ -35,9 +42,34 @@ async function initialize(): Promise<void> {
   return initializationPromise;
 }
 
-// TODO: Implement validators in https://github.com/paranext/paranext-core/issues/511
-async function isValid(): Promise<boolean> {
-  return true;
+async function isValid<ProjectSettingName extends ProjectSettingNames>(
+  key: ProjectSettingName,
+  newValue: ProjectSettingTypes[ProjectSettingName],
+  currentValue: ProjectSettingTypes[ProjectSettingName],
+  projectType: ProjectTypes,
+  allChanges?: SimultaneousProjectSettingsChanges,
+): Promise<boolean> {
+  if (key in coreProjectSettingsValidators) {
+    const projectSettingValidator = coreProjectSettingsValidators[key];
+    if (projectSettingValidator)
+      return projectSettingValidator(newValue, currentValue, allChanges ?? {}, projectType);
+    // If key exists in coreProjectSettingsValidators but there is no validator, let the change go through
+    return true;
+  }
+  try {
+    return networkService.request(
+      serializeRequestType(CATEGORY_EXTENSION_PROJECT_SETTING_VALIDATOR, key),
+      newValue,
+      currentValue,
+      allChanges ?? {},
+      projectType,
+    );
+  } catch (error) {
+    // If there is no validator just let the change go through
+    const missingValidatorMsg = `No handler was found to process the request of type`;
+    if (includes(`${error}`, missingValidatorMsg)) return true;
+    throw error;
+  }
 }
 
 async function getDefault<ProjectSettingName extends ProjectSettingNames>(
@@ -55,9 +87,11 @@ async function getDefault<ProjectSettingName extends ProjectSettingNames>(
   return projectSettingInfo.default;
 }
 
+const { registerValidator } = projectSettingsServiceObjectToProxy;
 const projectSettingsService: IProjectSettingsService = {
   isValid,
   getDefault,
+  registerValidator,
 };
 
 /** This is an internal-only export for testing purposes, and should not be used in development */
diff --git a/src/shared/services/project-settings.service-model.ts b/src/shared/services/project-settings.service-model.ts
index 14ee71c864..e0eef065db 100644
--- a/src/shared/services/project-settings.service-model.ts
+++ b/src/shared/services/project-settings.service-model.ts
@@ -1,4 +1,24 @@
+import { serializeRequestType } from '@shared/utils/util';
+import * as networkService from '@shared/services/network.service';
 import { ProjectSettingNames, ProjectSettingTypes, ProjectTypes } from 'papi-shared-types';
+import { UnsubscriberAsync } from 'platform-bible-utils';
+
+/** Name prefix for registered commands that call project settings validators */
+export const CATEGORY_EXTENSION_PROJECT_SETTING_VALIDATOR = 'extensionProjectSettingValidator';
+
+export const projectSettingsServiceNetworkObjectName = 'ProjectSettingsService';
+export const projectSettingsServiceObjectToProxy = Object.freeze({
+  /** JSDOC DESTINATION projectSettingsServiceRegisterValidator */
+  registerValidator: async <ProjectSettingName extends ProjectSettingNames>(
+    key: ProjectSettingName,
+    validator: ProjectSettingValidator<ProjectSettingName>,
+  ): Promise<UnsubscriberAsync> => {
+    return networkService.registerRequestHandler(
+      serializeRequestType(CATEGORY_EXTENSION_PROJECT_SETTING_VALIDATOR, key),
+      validator,
+    );
+  },
+});
 
 /**
  * JSDOC SOURCE projectSettingsService
@@ -22,11 +42,11 @@ export interface IProjectSettingsService {
    * @returns `true` if change is valid, `false` otherwise
    */
   isValid<ProjectSettingName extends ProjectSettingNames>(
+    key: ProjectSettingName,
     newValue: ProjectSettingTypes[ProjectSettingName],
     currentValue: ProjectSettingTypes[ProjectSettingName],
-    key: ProjectSettingName,
-    allChanges: SimultaneousProjectSettingsChanges,
     projectType: ProjectTypes,
+    allChanges?: SimultaneousProjectSettingsChanges,
   ): Promise<boolean>;
   /**
    * Gets default value for a project setting
@@ -45,6 +65,19 @@ export interface IProjectSettingsService {
     key: ProjectSettingName,
     projectType: ProjectTypes,
   ): Promise<ProjectSettingTypes[ProjectSettingName]>;
+  /**
+   * JSDOC SOURCE projectSettingsServiceRegisterValidator
+   *
+   * Registers a function that validates whether a new project setting value is allowed to be set.
+   *
+   * @param key The string id of the setting to validate
+   * @param validator Function to call to validate the new setting value
+   * @returns Unsubscriber that should be called whenever the providing extension is deactivated
+   */
+  registerValidator<ProjectSettingName extends ProjectSettingNames>(
+    key: ProjectSettingName,
+    validatorCallback: ProjectSettingValidator<ProjectSettingName>,
+  ): Promise<UnsubscriberAsync>;
 }
 
 /**
@@ -61,5 +94,18 @@ export type SimultaneousProjectSettingsChanges = {
     currentValue: ProjectSettingTypes[ProjectSettingName];
   };
 };
+/** Function that validates whether a new project setting value should be allowed to be set */
+export type ProjectSettingValidator<ProjectSettingName extends ProjectSettingNames> = (
+  newValue: ProjectSettingTypes[ProjectSettingName],
+  currentValue: ProjectSettingTypes[ProjectSettingName],
+  allChanges: SimultaneousProjectSettingsChanges,
+  projectType: ProjectTypes,
+) => Promise<boolean>;
 
-export const projectSettingsServiceNetworkObjectName = 'ProjectSettingsService';
+/**
+ * Validators for all project settings. Keys are setting keys, values are functions to validate new
+ * settings
+ */
+export type AllProjectSettingsValidators = {
+  [ProjectSettingName in ProjectSettingNames]: ProjectSettingValidator<ProjectSettingName>;
+};
diff --git a/src/shared/services/project-settings.service.ts b/src/shared/services/project-settings.service.ts
index 5040d78a06..b9fec0d3f9 100644
--- a/src/shared/services/project-settings.service.ts
+++ b/src/shared/services/project-settings.service.ts
@@ -1,8 +1,10 @@
 import {
   projectSettingsServiceNetworkObjectName,
   IProjectSettingsService,
+  projectSettingsServiceObjectToProxy,
 } from '@shared/services/project-settings.service-model';
 import networkObjectService from '@shared/services/network-object.service';
+import { createSyncProxyForAsyncObject } from 'platform-bible-utils';
 
 let networkObject: IProjectSettingsService;
 let initializationPromise: Promise<void>;
@@ -31,15 +33,9 @@ async function initialize(): Promise<void> {
   return initializationPromise;
 }
 
-const projectSettingsService: IProjectSettingsService = {
-  async getDefault(key, projectType) {
-    await initialize();
-    return networkObject.getDefault(key, projectType);
-  },
-  async isValid(newValue, currentValue, key, allChanges, projectType) {
-    await initialize();
-    return networkObject.isValid(newValue, currentValue, key, allChanges, projectType);
-  },
-};
+const projectSettingsService = createSyncProxyForAsyncObject<IProjectSettingsService>(async () => {
+  await initialize();
+  return networkObject;
+}, projectSettingsServiceObjectToProxy);
 
 export default projectSettingsService;