diff --git a/CHANGELOG.md b/CHANGELOG.md
index d6a363a9e1..345fe48283 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -37,6 +37,7 @@ For experimental package changes, see the [experimental CHANGELOG](experimental/
 
 * fix(core): added falsy check to make otel core work with browser where webpack config had process as false or null [#3613](https://github.com/open-telemetry/opentelemetry-js/issues/3613) @ravindra-dyte
 * fix(instrumentation-http): include query params in http.target [#3646](https://github.com/open-telemetry/opentelemetry-js/pull/3646) @kobi-co
+* fix(sdk-metrics): merge uncollected delta accumulations [#3667](https://github.com/open-telemetry/opentelemetry-js/pull/3667) @legendecas
 
 ### :books: (Refine Doc)
 
diff --git a/packages/sdk-metrics/src/state/DeltaMetricProcessor.ts b/packages/sdk-metrics/src/state/DeltaMetricProcessor.ts
index 3e6cb5eefc..e9b16f0e03 100644
--- a/packages/sdk-metrics/src/state/DeltaMetricProcessor.ts
+++ b/packages/sdk-metrics/src/state/DeltaMetricProcessor.ts
@@ -57,6 +57,7 @@ export class DeltaMetricProcessor<T extends Maybe<Accumulation>> {
           this._aggregator.createAccumulation(collectionTime);
         accumulation?.record(value);
         let delta = accumulation;
+        // Diff with recorded cumulative memo.
         if (this._cumulativeMemoStorage.has(attributes, hashCode)) {
           // has() returned true, previous is present.
           // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
@@ -66,6 +67,16 @@ export class DeltaMetricProcessor<T extends Maybe<Accumulation>> {
           )!;
           delta = this._aggregator.diff(previous, accumulation);
         }
+        // Merge with uncollected active delta.
+        if (this._activeCollectionStorage.has(attributes, hashCode)) {
+          // has() returned true, previous is present.
+          // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+          const active = this._activeCollectionStorage.get(
+            attributes,
+            hashCode
+          )!;
+          delta = this._aggregator.merge(active, delta);
+        }
 
         // Save the current record and the delta record.
         this._cumulativeMemoStorage.set(attributes, accumulation, hashCode);
diff --git a/packages/sdk-metrics/test/regression/two-metric-readers-async-instrument.test.ts b/packages/sdk-metrics/test/regression/two-metric-readers-async-instrument.test.ts
new file mode 100644
index 0000000000..a1301be0ec
--- /dev/null
+++ b/packages/sdk-metrics/test/regression/two-metric-readers-async-instrument.test.ts
@@ -0,0 +1,65 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as assert from 'assert';
+import { DataPointType, MeterProvider, MetricReader } from '../../src';
+import { TestDeltaMetricReader } from '../export/TestMetricReader';
+import { assertDataPoint, assertMetricData } from '../util';
+
+// https://github.com/open-telemetry/opentelemetry-js/issues/3664
+
+describe('two-metric-readers-async-instrument', () => {
+  it('both metric readers should collect metrics', async () => {
+    const meterProvider = new MeterProvider();
+    const reader1 = new TestDeltaMetricReader();
+    const reader2 = new TestDeltaMetricReader();
+
+    meterProvider.addMetricReader(reader1);
+    meterProvider.addMetricReader(reader2);
+
+    const meter = meterProvider.getMeter('my-meter');
+
+    let counter = 1;
+    const asyncUpDownCounter = meter.createObservableUpDownCounter(
+      'my_async_updowncounter'
+    );
+    asyncUpDownCounter.addCallback(observableResult => {
+      observableResult.observe(counter);
+    });
+
+    await assertCollection(reader1, 1);
+    await assertCollection(reader2, 1);
+
+    counter = 10;
+    await assertCollection(reader1, 9);
+    await assertCollection(reader2, 9);
+
+    async function assertCollection(reader: MetricReader, value: number) {
+      const { errors, resourceMetrics } = await reader.collect();
+      assert.strictEqual(errors.length, 0);
+
+      // Collected only one Metric.
+      assert.strictEqual(resourceMetrics.scopeMetrics.length, 1);
+      assert.strictEqual(resourceMetrics.scopeMetrics[0].metrics.length, 1);
+      const metric = resourceMetrics.scopeMetrics[0].metrics[0];
+
+      assertMetricData(metric, DataPointType.SUM, {
+        name: 'my_async_updowncounter',
+      });
+      assertDataPoint(metric.dataPoints[0], {}, value);
+    }
+  });
+});
diff --git a/packages/sdk-metrics/test/state/DeltaMetricProcessor.test.ts b/packages/sdk-metrics/test/state/DeltaMetricProcessor.test.ts
index d2d69f8432..ec0a3d6fff 100644
--- a/packages/sdk-metrics/test/state/DeltaMetricProcessor.test.ts
+++ b/packages/sdk-metrics/test/state/DeltaMetricProcessor.test.ts
@@ -100,6 +100,26 @@ describe('DeltaMetricProcessor', () => {
         assert.strictEqual(accumulation?.toPointValue(), 11);
       }
     });
+
+    it('should merge with active delta of accumulations', () => {
+      const metricProcessor = new DeltaMetricProcessor(new SumAggregator(true));
+
+      {
+        const measurements = new AttributeHashMap<number>();
+        measurements.set({}, 10);
+        metricProcessor.batchCumulate(measurements, [0, 0]);
+      }
+
+      {
+        const measurements = new AttributeHashMap<number>();
+        measurements.set({}, 20);
+        metricProcessor.batchCumulate(measurements, [1, 1]);
+      }
+
+      const accumulations = metricProcessor.collect();
+      const accumulation = accumulations.get({});
+      assert.strictEqual(accumulation?.toPointValue(), 20);
+    });
   });
 
   describe('collect', () => {