diff --git a/CHANGELOG.md b/CHANGELOG.md
index 37741608b4..4f41435022 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 * Added `Realm.Sync.Session.reconnect()` to help force a reconnection to Atlas Device Sync. ([#6123](https://github.com/realm/realm-js/issues/6123))
 
 ### Fixed
+* Fixed values of properties being replaced by default value when updating. ([#6129](https://github.com/realm/realm-js/issues/6129), since v12.0.0)
 * Fixed that value for `Realm.schemaVersion` wasn't propagated correctly for non-existing files. ([#6119](https://github.com/realm/realm-js/issues/6119), since v12.0.0)
 
 ### Compatibility
diff --git a/integration-tests/tests/src/tests/objects.ts b/integration-tests/tests/src/tests/objects.ts
index 0946458aae..4eec3cebbc 100644
--- a/integration-tests/tests/src/tests/objects.ts
+++ b/integration-tests/tests/src/tests/objects.ts
@@ -1335,4 +1335,38 @@ describe("Realm.Object", () => {
       expect(objFromKey?.age).equals(7);
     });
   });
+
+  // from https://github.com/realm/realm-js/issues/6129
+  describe("Primary key and property with default value", () => {
+    const PrimaryAndDefaultSchema: Realm.ObjectSchema = {
+      name: "MySchema",
+      primaryKey: "id",
+      properties: {
+        id: { type: "int" },
+        fieldOne: { type: "string" },
+        fieldTwo: { type: "string", default: "DEFAULT_VALUE" },
+      },
+    };
+    openRealmBeforeEach({ schema: [PrimaryAndDefaultSchema] });
+
+    for (const updateMode of [Realm.UpdateMode.All, Realm.UpdateMode.Modified]) {
+      it(`updates properties (updateMode = ${updateMode})`, async function (this: Mocha.Context & RealmContext) {
+        this.realm.write(() => {
+          this.realm.create(PrimaryAndDefaultSchema.name, {
+            id: 1337,
+            fieldOne: "SOME_VALUE",
+            fieldTwo: "NOT_DEFAULT_VALUE",
+          });
+        });
+
+        this.realm.write(() => {
+          this.realm.create(PrimaryAndDefaultSchema.name, { id: 1337, fieldOne: "SOME_OTHER_VALUE" }, updateMode);
+        });
+
+        const obj = this.realm.objectForPrimaryKey(PrimaryAndDefaultSchema.name, 1337);
+        expect(obj?.fieldOne).equals("SOME_OTHER_VALUE");
+        expect(obj?.fieldTwo).equals("NOT_DEFAULT_VALUE");
+      });
+    }
+  });
 });
diff --git a/packages/realm/src/Object.ts b/packages/realm/src/Object.ts
index 85be7f37f6..596cfb7e22 100644
--- a/packages/realm/src/Object.ts
+++ b/packages/realm/src/Object.ts
@@ -232,14 +232,15 @@ export class RealmObject<T = DefaultObject, RequiredProperties extends keyof Omi
           result[propertyName] = propertyValue;
         }
       } else {
-        if (typeof defaultValue !== "undefined") {
-          result[propertyName] = typeof defaultValue === "function" ? defaultValue() : defaultValue;
-        } else if (
-          !(property.type & binding.PropertyType.Collection) &&
-          !(property.type & binding.PropertyType.Nullable) &&
-          created
-        ) {
-          throw new Error(`Missing value for property '${propertyName}'`);
+        if (created) {
+          if (typeof defaultValue !== "undefined") {
+            result[propertyName] = typeof defaultValue === "function" ? defaultValue() : defaultValue;
+          } else if (
+            !(property.type & binding.PropertyType.Collection) &&
+            !(property.type & binding.PropertyType.Nullable)
+          ) {
+            throw new Error(`Missing value for property '${propertyName}'`);
+          }
         }
       }
     }