diff --git a/libraries/muxer/src/main/java/androidx/media3/muxer/Boxes.java b/libraries/muxer/src/main/java/androidx/media3/muxer/Boxes.java
index 3494b62a113..d036a35ad61 100644
--- a/libraries/muxer/src/main/java/androidx/media3/muxer/Boxes.java
+++ b/libraries/muxer/src/main/java/androidx/media3/muxer/Boxes.java
@@ -742,6 +742,9 @@ public static ByteBuffer codecSpecificBox(Format format) {
         return damrBox(/* mode= */ (short) 0x83FF); // mode set: all enabled for AMR-WB
       case MimeTypes.AUDIO_OPUS:
         return dOpsBox(format);
+      case MimeTypes.AUDIO_E_AC3:
+      case MimeTypes.AUDIO_E_AC3_JOC:
+        return dec3Box(format);
       case MimeTypes.AUDIO_RAW:
         return ByteBuffer.allocate(0); // No codec specific box for raw audio.
       case MimeTypes.VIDEO_H263:
@@ -1590,6 +1593,19 @@ private static ByteBuffer av1CBox(Format format) {
     return BoxUtils.wrapIntoBox("av1C", ByteBuffer.wrap(csd0));
   }
 
+  /**
+   * Returns the dec3 box (EC3SpecificBox) for E-AC-3 / E-AC-3 JOC (Dolby Atmos) audio.
+   *
+   * <p>The dec3 box body is the codec-specific configuration, carried verbatim in csd-0. Stream-copy
+   * callers must populate {@code format.initializationData} with the source track's raw dec3 payload.
+   */
+  private static ByteBuffer dec3Box(Format format) {
+    checkArgument(
+        !format.initializationData.isEmpty(), "csd-0 (dec3 payload) not found for dec3 box.");
+    byte[] csd0 = format.initializationData.get(0);
+    return BoxUtils.wrapIntoBox("dec3", ByteBuffer.wrap(csd0));
+  }
+
   /** Returns a dvcC/dvwC/dvvC vision box which will be included in dolby vision box. */
   private static ByteBuffer doviBox(int profile, byte[] csd) {
     checkArgument(csd.length > 0, "csd is empty for dovi box.");
@@ -1792,6 +1808,9 @@ private static String codecSpecificFourcc(Format format) {
         return "s263";
       case MimeTypes.AUDIO_OPUS:
         return "Opus";
+      case MimeTypes.AUDIO_E_AC3:
+      case MimeTypes.AUDIO_E_AC3_JOC:
+        return "ec-3";
       case MimeTypes.AUDIO_RAW:
         if (format.pcmEncoding == C.ENCODING_PCM_16BIT) {
           return "sowt";
diff --git a/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Muxer.java b/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Muxer.java
index 9ccf0320165..aee1d88b727 100644
--- a/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Muxer.java
+++ b/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Muxer.java
@@ -178,7 +178,9 @@ public FragmentedMp4Muxer build() {
           MimeTypes.AUDIO_AMR_WB,
           MimeTypes.AUDIO_OPUS,
           MimeTypes.AUDIO_VORBIS,
-          MimeTypes.AUDIO_RAW);
+          MimeTypes.AUDIO_RAW,
+          MimeTypes.AUDIO_E_AC3,
+          MimeTypes.AUDIO_E_AC3_JOC);
 
   // LINT.ThenChange(Boxes.java:codec_specific_boxes)
 
diff --git a/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Muxer.java b/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Muxer.java
index 7670afeeefe..9917d5dd5f1 100644
--- a/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Muxer.java
+++ b/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Muxer.java
@@ -404,7 +404,9 @@ public Mp4Muxer build() {
           MimeTypes.AUDIO_AMR_WB,
           MimeTypes.AUDIO_OPUS,
           MimeTypes.AUDIO_VORBIS,
-          MimeTypes.AUDIO_RAW);
+          MimeTypes.AUDIO_RAW,
+          MimeTypes.AUDIO_E_AC3,
+          MimeTypes.AUDIO_E_AC3_JOC);
 
   // LINT.ThenChange(Boxes.java:codec_specific_boxes)
 
diff --git a/libraries/muxer/src/test/java/androidx/media3/muxer/BoxesTest.java b/libraries/muxer/src/test/java/androidx/media3/muxer/BoxesTest.java
index 562b81991d4..805b0f9a999 100644
--- a/libraries/muxer/src/test/java/androidx/media3/muxer/BoxesTest.java
+++ b/libraries/muxer/src/test/java/androidx/media3/muxer/BoxesTest.java
@@ -400,6 +400,63 @@ public void createAudioSampleEntryBox_forVorbis_matchesExpected() throws Excepti
         MuxerTestUtil.getExpectedMp4DumpFilePath("audio_sample_entry_box_vorbis"));
   }
 
+  @Test
+  public void createCodecSpecificBox_forEAc3_wrapsDec3PayloadVerbatim() {
+    byte[] payload = new byte[] {0x0F, (byte) 0x80, 0x00};
+    Format format =
+        FAKE_AUDIO_FORMAT
+            .buildUpon()
+            .setSampleMimeType(MimeTypes.AUDIO_E_AC3)
+            .setInitializationData(ImmutableList.of(payload))
+            .build();
+
+    ByteBuffer box = Boxes.codecSpecificBox(format);
+
+    // Box layout: 4 bytes size + 4 bytes "dec3" type + payload bytes.
+    assertThat(box.remaining()).isEqualTo(8 + payload.length);
+    byte[] typeBytes = new byte[4];
+    box.position(4);
+    box.get(typeBytes);
+    assertThat(new String(typeBytes)).isEqualTo("dec3");
+    byte[] actual = new byte[payload.length];
+    box.get(actual);
+    assertThat(actual).isEqualTo(payload);
+  }
+
+  @Test
+  public void createCodecSpecificBox_forEAc3Joc_wrapsDec3PayloadVerbatim() {
+    byte[] payload = new byte[] {0x0F, (byte) 0x80, 0x00};
+    Format format =
+        FAKE_AUDIO_FORMAT
+            .buildUpon()
+            .setSampleMimeType(MimeTypes.AUDIO_E_AC3_JOC)
+            .setInitializationData(ImmutableList.of(payload))
+            .build();
+
+    ByteBuffer box = Boxes.codecSpecificBox(format);
+
+    assertThat(box.remaining()).isEqualTo(8 + payload.length);
+    byte[] typeBytes = new byte[4];
+    box.position(4);
+    box.get(typeBytes);
+    assertThat(new String(typeBytes)).isEqualTo("dec3");
+    byte[] actual = new byte[payload.length];
+    box.get(actual);
+    assertThat(actual).isEqualTo(payload);
+  }
+
+  @Test
+  public void createCodecSpecificBox_forEAc3WithNoCsd_throws() {
+    Format format =
+        FAKE_AUDIO_FORMAT
+            .buildUpon()
+            .setSampleMimeType(MimeTypes.AUDIO_E_AC3)
+            .setInitializationData(ImmutableList.of())
+            .build();
+
+    assertThrows(IllegalArgumentException.class, () -> Boxes.codecSpecificBox(format));
+  }
+
   @Test
   public void createAudioSampleEntryBox_withUnknownAudioFormat_throws() {
     // The audio format contains an unknown MIME type.