diff --git a/DESCRIPTION b/DESCRIPTION
index 765cd22c5..71376e7e7 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -112,6 +112,7 @@ Suggests:
     miceadds,
     mlogit,
     mlr3verse,
+    mmrm (>= 0.3.12),
     modelbased,
     modelsummary,
     nlme,
@@ -246,6 +247,7 @@ Collate:
     'methods_mlm.R'
     'methods_mlogit.R'
     'methods_mlr3.R'
+    'methods_mmrm.R'
     'methods_nlme.R'
     'methods_ordinal.R'
     'methods_plm.R'
diff --git a/NAMESPACE b/NAMESPACE
index 9c2e7b48f..494703309 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -21,6 +21,7 @@ S3method(get_coef,lmerModLmerTest)
 S3method(get_coef,mblogit)
 S3method(get_coef,merMod)
 S3method(get_coef,mlm)
+S3method(get_coef,mmrm)
 S3method(get_coef,multinom)
 S3method(get_coef,nls)
 S3method(get_coef,polr)
@@ -76,6 +77,7 @@ S3method(get_predict,mblogit)
 S3method(get_predict,merMod)
 S3method(get_predict,mhurdle)
 S3method(get_predict,mlogit)
+S3method(get_predict,mmrm)
 S3method(get_predict,model_fit)
 S3method(get_predict,multinom)
 S3method(get_predict,ols)
@@ -104,6 +106,7 @@ S3method(get_vcov,glimML)
 S3method(get_vcov,glmmTMB)
 S3method(get_vcov,gsm)
 S3method(get_vcov,mhurdle)
+S3method(get_vcov,mmrm)
 S3method(get_vcov,model_fit)
 S3method(get_vcov,orm)
 S3method(get_vcov,pstpm2)
@@ -160,6 +163,7 @@ S3method(set_coef,lmerMod)
 S3method(set_coef,lmerModLmerTest)
 S3method(set_coef,merMod)
 S3method(set_coef,mlm)
+S3method(set_coef,mmrm)
 S3method(set_coef,model_fit)
 S3method(set_coef,multinom)
 S3method(set_coef,nls)
diff --git a/NEWS.md b/NEWS.md
index 2840055b7..481bc60dc 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -9,9 +9,10 @@ Breaking changes:
 
 New models supported:
 
-* `stpm2`, `pstpm2`, `gsm`, and `aft` models from `rstpm2`. Thanks to @aghaynes and @mclements.
+* `rstpm2` package: support for `stpm2`, `pstpm2`, `gsm`, and `aft` models. Thanks to @mclements for contribution #1188 and @aghayes for useful comments.
 * `glm_weightit`, `coxph_weightit`, `multinom_weightit`, and `ordinal_weightit` models from `Weightit`. Thanks to @ngreifer.
 * `glmmgee` from the `glmtoolbox` package. Thanks to @adrianolszewski for the request and @lhvanegasp for help with implementation.
+* `mmrm` package for mixed effects modelling. Thanks to @kkmann for contribution #1000, as well as @clarkliming and @lang-benjamin for useful comments.
 
 New features:
 
diff --git a/R/methods_mmrm.R b/R/methods_mmrm.R
new file mode 100644
index 000000000..3c7e77967
--- /dev/null
+++ b/R/methods_mmrm.R
@@ -0,0 +1,48 @@
+#' @include set_coef.R
+#' @rdname set_coef
+#' @keywords internal
+#' @export
+set_coef.mmrm <- function(model, coefs, ...) {
+  model$beta_est  <- coefs
+  return(model)
+}
+
+
+#' @include get_coef.R
+#' @rdname get_coef
+#' @keywords internal
+#' @export
+get_coef.mmrm <- function(model, ...) {
+  return(coef(model, ...))
+}
+
+
+#' @include get_vcov.R
+#' @rdname get_vcov
+#' @keywords internal
+#' @export
+get_vcov.mmrm <- function(model, ...) {
+  return(vcov(model, ...))
+}
+
+
+#' @rdname get_predict
+#' @keywords internal
+#' @export
+get_predict.mmrm <- function(model, newdata = model$data, type = "response", ...) {
+  type <- match.arg(type)
+  res <- data.frame(
+    rowid = seq_len(nrow(newdata)),
+    estimate = predict(model, newdata = newdata, type = type, conditional = FALSE)
+  )
+  return(res)
+}
+
+
+#' @include sanity_model.R
+#' @rdname sanitize_model_specific
+#' @keywords internal
+sanitize_model_specific.mmrm <- function(model, ...) {
+    insight::check_if_installed("mmrm", minimum_version = "0.3.11.9000")
+    return(model)
+}
diff --git a/R/sanity_model.R b/R/sanity_model.R
index 09655cda5..808f67592 100644
--- a/R/sanity_model.R
+++ b/R/sanity_model.R
@@ -88,6 +88,7 @@ sanity_model_supported_class <- function(model) {
         "mhurdle",
         "mira",
         "mlogit",
+        "mmrm",
         "model_fit",
         c("multinom", "nnet"),
         "multinom_weightit",
diff --git a/inst/tinytest/test-pkg-mmrm.R b/inst/tinytest/test-pkg-mmrm.R
new file mode 100644
index 000000000..c0ecacfaa
--- /dev/null
+++ b/inst/tinytest/test-pkg-mmrm.R
@@ -0,0 +1,17 @@
+source("helpers.R")
+using("marginaleffects")
+
+requiet("mmrm")
+
+fit <- mmrm(
+  formula = FEV1 ~ RACE + SEX + ARMCD * AVISIT + us(AVISIT | USUBJID),
+  data = fev_data
+)
+
+pre <- avg_predictions(fit, newdata = fev_data)
+expect_inherits(pre, "predictions")
+
+cmp <- avg_comparisons(fit, 
+    variables = "SEX",
+    newdata = fev_data)
+expect_inherits(cmp, "comparisons")
diff --git a/man/get_coef.Rd b/man/get_coef.Rd
index ab041b218..a43de16fa 100644
--- a/man/get_coef.Rd
+++ b/man/get_coef.Rd
@@ -26,6 +26,7 @@
 \alias{get_coef.mblogit}
 \alias{get_coef.gam}
 \alias{get_coef.mlm}
+\alias{get_coef.mmrm}
 \alias{get_coef.selection}
 \alias{get_coef.scam}
 \alias{get_coef.nls}
@@ -71,6 +72,8 @@ get_coef(model, ...)
 
 \method{get_coef}{mlm}(model, ...)
 
+\method{get_coef}{mmrm}(model, ...)
+
 \method{get_coef}{selection}(model, ...)
 
 \method{get_coef}{scam}(model, ...)
diff --git a/man/get_predict.Rd b/man/get_predict.Rd
index 9275573d9..046f3b77d 100644
--- a/man/get_predict.Rd
+++ b/man/get_predict.Rd
@@ -38,6 +38,7 @@
 \alias{get_predict.mhurdle}
 \alias{get_predict.mlogit}
 \alias{get_predict.Learner}
+\alias{get_predict.mmrm}
 \alias{get_predict.clm}
 \alias{get_predict.rq}
 \alias{get_predict.rms}
@@ -120,6 +121,8 @@ get_predict(model, newdata, type, ...)
 
 \method{get_predict}{Learner}(model, newdata, type = NULL, ...)
 
+\method{get_predict}{mmrm}(model, newdata = model$data, type = "response", ...)
+
 \method{get_predict}{clm}(model, newdata = insight::get_data(model), type = "prob", ...)
 
 \method{get_predict}{rq}(model, newdata = insight::get_data(model), type = NULL, ...)
diff --git a/man/get_vcov.Rd b/man/get_vcov.Rd
index e944e9360..d78823027 100644
--- a/man/get_vcov.Rd
+++ b/man/get_vcov.Rd
@@ -2,8 +2,8 @@
 % Please edit documentation in R/get_vcov.R, R/methods_MCMCglmm.R,
 %   R/methods_afex.R, R/methods_aod.R, R/methods_biglm.R, R/methods_brms.R,
 %   R/methods_dbarts.R, R/methods_gamlss.R, R/methods_glmmTMB.R,
-%   R/methods_mhurdle.R, R/methods_mlr3.R, R/methods_rms.R, R/methods_rstpm2.R,
-%   R/methods_scam.R, R/methods_tidymodels.R
+%   R/methods_mhurdle.R, R/methods_mlr3.R, R/methods_mmrm.R, R/methods_rms.R,
+%   R/methods_rstpm2.R, R/methods_scam.R, R/methods_tidymodels.R
 \name{get_vcov}
 \alias{get_vcov}
 \alias{get_vcov.default}
@@ -17,6 +17,7 @@
 \alias{get_vcov.glmmTMB}
 \alias{get_vcov.mhurdle}
 \alias{get_vcov.Learner}
+\alias{get_vcov.mmrm}
 \alias{get_vcov.orm}
 \alias{get_vcov.stpm2}
 \alias{get_vcov.pstpm2}
@@ -51,6 +52,8 @@ get_vcov(model, ...)
 
 \method{get_vcov}{Learner}(model, ...)
 
+\method{get_vcov}{mmrm}(model, ...)
+
 \method{get_vcov}{orm}(model, vcov = NULL, ...)
 
 \method{get_vcov}{stpm2}(model, ...)
diff --git a/man/sanitize_model_specific.Rd b/man/sanitize_model_specific.Rd
index 744c4a1d2..09e28cc8e 100644
--- a/man/sanitize_model_specific.Rd
+++ b/man/sanitize_model_specific.Rd
@@ -1,7 +1,7 @@
 % Generated by roxygen2: do not edit by hand
 % Please edit documentation in R/methods_aod.R, R/methods_betareg.R,
 %   R/sanity_model.R, R/methods_brms.R, R/methods_dbarts.R, R/methods_glmmTMB.R,
-%   R/methods_lme4.R, R/methods_mclogit.R, R/methods_mlogit.R,
+%   R/methods_lme4.R, R/methods_mclogit.R, R/methods_mlogit.R, R/methods_mmrm.R,
 %   R/methods_ordinal.R, R/methods_plm.R, R/methods_quantreg.R,
 %   R/methods_survey.R
 \name{sanitize_model_specific.glimML}
@@ -15,6 +15,7 @@
 \alias{sanitize_model_specific.merMod}
 \alias{sanitize_model_specific.mblogit}
 \alias{sanitize_model_specific.mlogit}
+\alias{sanitize_model_specific.mmrm}
 \alias{sanitize_model_specific.clm}
 \alias{sanitize_model_specific.plm}
 \alias{sanitize_model_specific.rqs}
@@ -47,6 +48,8 @@ sanitize_model_specific(model, ...)
 
 \method{sanitize_model_specific}{mlogit}(model, newdata, ...)
 
+\method{sanitize_model_specific}{mmrm}(model, ...)
+
 \method{sanitize_model_specific}{clm}(model, ...)
 
 \method{sanitize_model_specific}{plm}(model, ...)
diff --git a/man/set_coef.Rd b/man/set_coef.Rd
index 32349355f..d24e1e41e 100644
--- a/man/set_coef.Rd
+++ b/man/set_coef.Rd
@@ -32,6 +32,7 @@
 \alias{set_coef.lmerModLmerTest}
 \alias{set_coef.lmerMod}
 \alias{set_coef.mlm}
+\alias{set_coef.mmrm}
 \alias{set_coef.lme}
 \alias{set_coef.hurdle}
 \alias{set_coef.zeroinfl}
@@ -97,6 +98,8 @@ set_coef(model, coefs, ...)
 
 \method{set_coef}{mlm}(model, coefs, ...)
 
+\method{set_coef}{mmrm}(model, coefs, ...)
+
 \method{set_coef}{lme}(model, coefs, ...)
 
 \method{set_coef}{hurdle}(model, coefs, ...)