diff --git a/dlls/mf/samplegrabber.c b/dlls/mf/samplegrabber.c
index af9666c86c8..c3dc2e1b7f3 100644
--- a/dlls/mf/samplegrabber.c
+++ b/dlls/mf/samplegrabber.c
@@ -862,6 +862,8 @@ static HRESULT WINAPI sample_grabber_sink_GetCharacteristics(IMFMediaSink *iface
         return MF_E_SHUTDOWN;
 
     *flags = MEDIASINK_FIXED_STREAMS;
+    if (grabber->ignore_clock)
+        *flags |= MEDIASINK_RATELESS;
 
     return S_OK;
 }
diff --git a/dlls/mf/tests/mf.c b/dlls/mf/tests/mf.c
index 7f9edfe241c..830bb38259c 100644
--- a/dlls/mf/tests/mf.c
+++ b/dlls/mf/tests/mf.c
@@ -2166,6 +2166,32 @@ todo_wine
     refcount = IMFActivate_Release(activate);
     ok(!refcount, "Unexpected refcount %u.\n", refcount);
 
+    /* Rateless mode with MF_SAMPLEGRABBERSINK_IGNORE_CLOCK. */
+    hr = MFCreateMediaType(&media_type);
+    ok(hr == S_OK, "Failed to create media type, hr %#x.\n", hr);
+
+    hr = IMFMediaType_SetGUID(media_type, &MF_MT_MAJOR_TYPE, &MFMediaType_Audio);
+    ok(hr == S_OK, "Failed to set attribute, hr %#x.\n", hr);
+    hr = IMFMediaType_SetGUID(media_type, &MF_MT_SUBTYPE, &MFAudioFormat_PCM);
+    ok(hr == S_OK, "Failed to set attribute, hr %#x.\n", hr);
+
+    hr = MFCreateSampleGrabberSinkActivate(media_type, &grabber_callback, &activate);
+    ok(hr == S_OK, "Failed to create grabber activate, hr %#x.\n", hr);
+
+    hr = IMFActivate_SetUINT32(activate, &MF_SAMPLEGRABBERSINK_IGNORE_CLOCK, 1);
+    ok(hr == S_OK, "Failed to set attribute, hr %#x.\n", hr);
+
+    hr = IMFActivate_ActivateObject(activate, &IID_IMFMediaSink, (void **)&sink);
+    ok(hr == S_OK, "Failed to activate object, hr %#x.\n", hr);
+
+    hr = IMFMediaSink_GetCharacteristics(sink, &flags);
+    ok(hr == S_OK, "Failed to get sink flags, hr %#x.\n", hr);
+    ok(flags & MEDIASINK_RATELESS, "Unexpected flags %#x.\n", flags);
+
+    IMFMediaSink_Release(sink);
+    IMFActivate_Release(activate);
+    IMFMediaType_Release(media_type);
+
     hr = MFShutdown();
     ok(hr == S_OK, "Failed to shut down, hr %#x.\n", hr);
 }