diff --git a/lib/modules.nix b/lib/modules.nix
index 21ffbb9591c5..1adee44f7a33 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -1598,6 +1598,28 @@ let
     inherit priority content;
   };
 
+  /**
+    Applies a function to the value inside a definition,
+    preserving all surrounding properties (`mkForce`, `mkOrder`, `mkIf`, etc.).
+  */
+  mapDefinitionValue =
+    f: def:
+    if def ? _type then
+      if def._type == "merge" then
+        def // { contents = map (mapDefinitionValue f) def.contents; }
+      else if def._type == "if" then
+        def // { content = mapDefinitionValue f def.content; }
+      else if def._type == "override" then
+        def // { content = mapDefinitionValue f def.content; }
+      else if def._type == "order" then
+        def // { content = mapDefinitionValue f def.content; }
+      else if def._type == "definition" then
+        def // { value = mapDefinitionValue f def.value; }
+      else
+        f def
+    else
+      f def;
+
   mkBefore = mkOrder 500;
   defaultOrderPriority = 1000;
   mkAfter = mkOrder 1500;
@@ -2302,6 +2324,7 @@ private
     importApply
     importJSON
     importTOML
+    mapDefinitionValue
     mergeDefinitions
     mergeAttrDefinitionsWithPrio
     mergeOptionDecls # should be private?
diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index e2872dcafce9..021a7eca25b8 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -5114,4 +5114,96 @@ runTests {
     );
     expected = false;
   };
+
+  # mapDefinitionValue
+
+  testMapDefinitionValuePlain = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) 5;
+    expected = 6;
+  };
+
+  testMapDefinitionValueMkForce = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (lib.mkForce 5);
+    expected = lib.mkForce 6;
+  };
+
+  testMapDefinitionValueMkDefault = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (lib.mkDefault 5);
+    expected = lib.mkDefault 6;
+  };
+
+  testMapDefinitionValueMkOrder = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (lib.mkOrder 500 5);
+    expected = lib.mkOrder 500 6;
+  };
+
+  testMapDefinitionValueMkOverrideNested = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (lib.mkForce (lib.mkOrder 500 5));
+    expected = lib.mkForce (lib.mkOrder 500 6);
+  };
+
+  testMapDefinitionValueMkIf = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (lib.mkIf true 5);
+    expected = lib.mkIf true 6;
+  };
+
+  testMapDefinitionValueMkMerge = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (
+      lib.mkMerge [
+        5
+        10
+      ]
+    );
+    expected = lib.mkMerge [
+      6
+      11
+    ];
+  };
+
+  testMapDefinitionValueMkDefinition = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (
+      lib.mkDefinition {
+        file = "test";
+        value = 5;
+      }
+    );
+    expected = lib.mkDefinition {
+      file = "test";
+      value = 6;
+    };
+  };
+
+  testMapDefinitionValueDeep = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (lib.mkIf true (lib.mkForce (lib.mkOrder 500 5)));
+    expected = lib.mkIf true (lib.mkForce (lib.mkOrder 500 6));
+  };
+
+  testMapDefinitionValueAllNested = {
+    expr = lib.modules.mapDefinitionValue (x: x + 1) (
+      lib.mkMerge [
+        (lib.mkIf true (
+          lib.mkForce (
+            lib.mkOrder 500 (
+              lib.mkDefinition {
+                file = "test";
+                value = lib.mkBefore 5;
+              }
+            )
+          )
+        ))
+      ]
+    );
+    expected = lib.mkMerge [
+      (lib.mkIf true (
+        lib.mkForce (
+          lib.mkOrder 500 (
+            lib.mkDefinition {
+              file = "test";
+              value = lib.mkBefore 6;
+            }
+          )
+        )
+      ))
+    ];
+  };
 }