diff --git a/CHANGELOG.md b/CHANGELOG.md
index f1373312..e3257b06 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ Latest
 * [#239](https://github.com/cleverage/process-bundle/issues/239) Fix PropertyGetterTask: catch any `\Throwable`, so that the `TypeError` of the property accessor on a scalar input is set on the state (with the `property` error context) and handled according to the task `error_strategy` instead of escaping it. Update documentation, add tests.
 * [#241](https://github.com/cleverage/process-bundle/issues/241) Fix CsvWriterTask: a non-array input throws the documented `\UnexpectedValueException` instead of a `TypeError` when `headers` is `null`, and the CSV tasks release their file on `finalize()` so that the task can be executed again. Update documentation, add tests.
 * [#240](https://github.com/cleverage/process-bundle/issues/240) Fix FileMoverTask: with `autoincrement`, the suffix was computed on the whole path (a dot in a directory name was taken as the extension, making the move fail for a file without extension) and an existing numeric suffix of the file name was replaced (`report-2024.csv` became `report-1.csv`). The suffix is now appended to the file name only (`report-2024-1.csv`, `file-1`, `.env-1`). Update documentation, add tests.
+* [#243](https://github.com/cleverage/process-bundle/issues/243) Fix RecursivePropertySetterTransformer: a `\stdClass` item without the property was replaced by a copy in the output, so the input object was not modified; the property is now added to the item itself. Update documentation, add tests.
 
 v5.1
 -----
diff --git a/docs/reference/transformers/recursive_property_setter_transformer.md b/docs/reference/transformers/recursive_property_setter_transformer.md
index cad98ddc..c54f0920 100644
--- a/docs/reference/transformers/recursive_property_setter_transformer.md
+++ b/docs/reference/transformers/recursive_property_setter_transformer.md
@@ -56,6 +56,6 @@ Notes
 
 Keys of `set_properties` are property paths written with the PropertyAccessor on each item: use the `[key]` notation
 for array items and the `property` notation for objects. For `\stdClass` items, a property that cannot be written is
-added to the object.
+added to the object itself (a nested path such as `parent.id` is added as a `parent.id` property).
 
 Object items are modified in place, so the objects of the input are modified too.
diff --git a/src/Transformer/Object/RecursivePropertySetterTransformer.php b/src/Transformer/Object/RecursivePropertySetterTransformer.php
index 4a924666..1402b6aa 100644
--- a/src/Transformer/Object/RecursivePropertySetterTransformer.php
+++ b/src/Transformer/Object/RecursivePropertySetterTransformer.php
@@ -66,9 +66,8 @@ public function transform(mixed $value, array $options = []): mixed
                     $this->accessor->setValue($item, $propertyPath, $propertyValue);
                 } catch (NoSuchPropertyException $e) {
                     if ($item instanceof \stdClass) {
-                        $item = (object) array_merge((array) $item, [
-                            $propertyPath => $propertyValue,
-                        ]);
+                        // Add the property to the item itself, to modify it in place
+                        $item->{$propertyPath} = $propertyValue;
                     } else {
                         throw $e;
                     }
diff --git a/tests/Transformer/Object/RecursivePropertySetterTransformerTest.php b/tests/Transformer/Object/RecursivePropertySetterTransformerTest.php
index 9914ef76..4c49784d 100644
--- a/tests/Transformer/Object/RecursivePropertySetterTransformerTest.php
+++ b/tests/Transformer/Object/RecursivePropertySetterTransformerTest.php
@@ -84,6 +84,39 @@ public function testMissingPropertyIsAddedToStdClassItems(): void
         self::assertEquals([(object) ['label' => 'A', 'parentId' => 7]], $result);
     }
 
+    public function testMissingPropertyIsAddedInPlaceToStdClassItems(): void
+    {
+        $transformer = $this->createTransformer();
+        $options = $this->resolveOptions($transformer, [
+            'iterator' => 'items',
+            'set_properties' => ['parentId' => 'id'],
+        ]);
+        $item = (object) ['label' => 'A'];
+
+        $result = $transformer->transform((object) ['id' => 7, 'items' => [$item]], $options);
+
+        self::assertIsArray($result);
+        self::assertSame($item, $result[0]);
+        self::assertSame(7, $item->parentId);
+    }
+
+    public function testMissingNestedPropertyPathIsAddedAsPropertyToStdClassItems(): void
+    {
+        $transformer = $this->createTransformer();
+        $options = $this->resolveOptions($transformer, [
+            'iterator' => 'items',
+            'set_properties' => ['parent.id' => 'id'],
+        ]);
+
+        $item = (object) ['label' => 'A'];
+
+        $result = $transformer->transform((object) ['id' => 7, 'items' => [$item]], $options);
+
+        self::assertIsArray($result);
+        self::assertSame($item, $result[0]);
+        self::assertEquals((object) ['label' => 'A', 'parent.id' => 7], $item);
+    }
+
     public function testTraversableCollectionIsSupported(): void
     {
         $transformer = $this->createTransformer();