diff --git a/RELEASENOTES.md b/RELEASENOTES.md
index 2992f9994e..631f0e8bdb 100644
--- a/RELEASENOTES.md
+++ b/RELEASENOTES.md
@@ -4,6 +4,9 @@
 
 *   Common Library:
 *   ExoPlayer:
+    *   Add source prefix to all `Format.id` fields generated from
+        `MergingMediaSource`. This helps to identify which source produced a
+        `Format` ([#883](https://github.com/androidx/media/issues/883)).
 *   Transformer:
 *   Track Selection:
 *   Extractors:
diff --git a/libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/MergingMediaPeriod.java b/libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/MergingMediaPeriod.java
index 6e910e4b4a..07ff893bfd 100644
--- a/libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/MergingMediaPeriod.java
+++ b/libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/MergingMediaPeriod.java
@@ -284,7 +284,17 @@ import java.util.List;
       int periodTrackGroupCount = periodTrackGroups.length;
       for (int j = 0; j < periodTrackGroupCount; j++) {
         TrackGroup childTrackGroup = periodTrackGroups.get(j);
-        TrackGroup mergedTrackGroup = childTrackGroup.copyWithId(i + ":" + childTrackGroup.id);
+        Format[] mergedFormats = new Format[childTrackGroup.length];
+        for (int k = 0; k < childTrackGroup.length; k++) {
+          Format originalFormat = childTrackGroup.getFormat(k);
+          mergedFormats[k] =
+              originalFormat
+                  .buildUpon()
+                  .setId(i + ":" + (originalFormat.id == null ? "" : originalFormat.id))
+                  .build();
+        }
+        TrackGroup mergedTrackGroup =
+            new TrackGroup(/* id= */ i + ":" + childTrackGroup.id, mergedFormats);
         childTrackGroupByMergedTrackGroup.put(mergedTrackGroup, childTrackGroup);
         trackGroupArray[trackGroupIndex++] = mergedTrackGroup;
       }
@@ -325,7 +335,7 @@ import java.util.List;
 
     @Override
     public Format getFormat(int index) {
-      return trackSelection.getFormat(index);
+      return trackGroup.getFormat(trackSelection.getIndexInTrackGroup(index));
     }
 
     @Override
@@ -335,7 +345,7 @@ import java.util.List;
 
     @Override
     public int indexOf(Format format) {
-      return trackSelection.indexOf(format);
+      return trackSelection.indexOf(trackGroup.indexOf(format));
     }
 
     @Override
@@ -355,7 +365,7 @@ import java.util.List;
 
     @Override
     public Format getSelectedFormat() {
-      return trackSelection.getSelectedFormat();
+      return trackGroup.getFormat(trackSelection.getSelectedIndexInTrackGroup());
     }
 
     @Override
diff --git a/libraries/exoplayer/src/test/java/androidx/media3/exoplayer/source/MergingMediaPeriodTest.java b/libraries/exoplayer/src/test/java/androidx/media3/exoplayer/source/MergingMediaPeriodTest.java
index 92bd17b894..3f3278f654 100644
--- a/libraries/exoplayer/src/test/java/androidx/media3/exoplayer/source/MergingMediaPeriodTest.java
+++ b/libraries/exoplayer/src/test/java/androidx/media3/exoplayer/source/MergingMediaPeriodTest.java
@@ -61,10 +61,10 @@ public final class MergingMediaPeriodTest {
                 /* timeOffsetUs= */ 0, /* singleSampleTimeUs= */ 0, childFormat21, childFormat22));
 
     assertThat(mergingMediaPeriod.getTrackGroups().length).isEqualTo(4);
-    assertThat(mergingMediaPeriod.getTrackGroups().get(0).getFormat(0)).isEqualTo(childFormat11);
-    assertThat(mergingMediaPeriod.getTrackGroups().get(1).getFormat(0)).isEqualTo(childFormat12);
-    assertThat(mergingMediaPeriod.getTrackGroups().get(2).getFormat(0)).isEqualTo(childFormat21);
-    assertThat(mergingMediaPeriod.getTrackGroups().get(3).getFormat(0)).isEqualTo(childFormat22);
+    assertThat(mergingMediaPeriod.getTrackGroups().get(0).getFormat(0).id).isEqualTo("0:1_1");
+    assertThat(mergingMediaPeriod.getTrackGroups().get(1).getFormat(0).id).isEqualTo("0:1_2");
+    assertThat(mergingMediaPeriod.getTrackGroups().get(2).getFormat(0).id).isEqualTo("1:2_1");
+    assertThat(mergingMediaPeriod.getTrackGroups().get(3).getFormat(0).id).isEqualTo("1:2_2");
   }
 
   @Test
@@ -199,6 +199,44 @@ public final class MergingMediaPeriodTest {
     assertThat(firstSelectionChild2).isEqualTo(secondSelectionChild2);
   }
 
+  @Test
+  public void selectTracks_forwardsSelectionsWithChildIdsToChildSources() throws Exception {
+    MergingMediaPeriod mergingMediaPeriod =
+        prepareMergingPeriod(
+            /* singleTrackGroup= */ true,
+            new MergingPeriodDefinition(
+                /* timeOffsetUs= */ 0, /* singleSampleTimeUs= */ 0, childFormat11, childFormat12),
+            new MergingPeriodDefinition(
+                /* timeOffsetUs= */ 0, /* singleSampleTimeUs= */ 0, childFormat21, childFormat22));
+    FakeMediaPeriodWithSelectionParameters childMediaPeriod1 =
+        (FakeMediaPeriodWithSelectionParameters) mergingMediaPeriod.getChildPeriod(0);
+    FakeMediaPeriodWithSelectionParameters childMediaPeriod2 =
+        (FakeMediaPeriodWithSelectionParameters) mergingMediaPeriod.getChildPeriod(1);
+
+    TrackGroupArray mergedTrackGroups = mergingMediaPeriod.getTrackGroups();
+    ExoTrackSelection[] selectionArray =
+        new ExoTrackSelection[] {
+          new FixedTrackSelection(mergedTrackGroups.get(0), /* track= */ 0),
+          new FixedTrackSelection(mergedTrackGroups.get(1), /* track= */ 1)
+        };
+
+    mergingMediaPeriod.selectTracks(
+        selectionArray,
+        /* mayRetainStreamFlags= */ new boolean[2],
+        /* streams= */ new SampleStream[2],
+        /* streamResetFlags= */ new boolean[2],
+        /* positionUs= */ 0);
+    ExoTrackSelection selectionChild1 = childMediaPeriod1.selectTracksSelections[0];
+    ExoTrackSelection selectionChild2 = childMediaPeriod2.selectTracksSelections[1];
+
+    assertThat(selectionChild1.getSelectedFormat()).isEqualTo(childFormat11);
+    assertThat(selectionChild2.getSelectedFormat()).isEqualTo(childFormat22);
+    assertThat(selectionChild1.getFormat(/* index= */ 0)).isEqualTo(childFormat11);
+    assertThat(selectionChild2.getFormat(/* index= */ 0)).isEqualTo(childFormat22);
+    assertThat(selectionChild1.indexOf(childFormat11)).isEqualTo(0);
+    assertThat(selectionChild2.indexOf(childFormat22)).isEqualTo(0);
+  }
+
   // https://github.com/google/ExoPlayer/issues/10930
   @Test
   public void selectTracks_withIdenticalFormats_selectsMatchingPeriod() throws Exception {
@@ -234,14 +272,24 @@ public final class MergingMediaPeriodTest {
 
   private MergingMediaPeriod prepareMergingPeriod(MergingPeriodDefinition... definitions)
       throws Exception {
+    return prepareMergingPeriod(/* singleTrackGroup= */ false, definitions);
+  }
+
+  private MergingMediaPeriod prepareMergingPeriod(
+      boolean singleTrackGroup, MergingPeriodDefinition... definitions) throws Exception {
     MediaPeriod[] mediaPeriods = new MediaPeriod[definitions.length];
     long[] timeOffsetsUs = new long[definitions.length];
     for (int i = 0; i < definitions.length; i++) {
       MergingPeriodDefinition definition = definitions[i];
       timeOffsetsUs[i] = definition.timeOffsetUs;
-      TrackGroup[] trackGroups = new TrackGroup[definition.formats.length];
-      for (int j = 0; j < definition.formats.length; j++) {
-        trackGroups[j] = new TrackGroup(definition.formats[j]);
+      TrackGroup[] trackGroups;
+      if (singleTrackGroup) {
+        trackGroups = new TrackGroup[] {new TrackGroup(definition.formats)};
+      } else {
+        trackGroups = new TrackGroup[definition.formats.length];
+        for (int j = 0; j < definition.formats.length; j++) {
+          trackGroups[j] = new TrackGroup(definition.formats[j]);
+        }
       }
       mediaPeriods[i] =
           new FakeMediaPeriodWithSelectionParameters(