diff --git a/media/mp4/box_reader.cc b/media/mp4/box_reader.cc
index 2d7689c76b..2c6fe5fad3 100644
--- a/media/mp4/box_reader.cc
+++ b/media/mp4/box_reader.cc
@@ -35,6 +35,10 @@ BoxReader* BoxReader::ReadTopLevelBox(const uint8* buf,
   if (!reader->ReadHeader(err))
     return NULL;
 
+  // We don't require the complete box to be available for MDAT box.
+  if (reader->type() == FOURCC_MDAT)
+    return reader.release();
+
   if (!IsValidTopLevelBox(reader->type())) {
     *err = true;
     return NULL;
diff --git a/media/mp4/box_reader.h b/media/mp4/box_reader.h
index 1a7261b31e..036124d9ae 100644
--- a/media/mp4/box_reader.h
+++ b/media/mp4/box_reader.h
@@ -30,7 +30,10 @@ class BoxReader : public BufferReader {
   /// @param buf_size indicates the size of the input buffer.
   /// @param[out] err is set to true if there was a stream-level error when
   ///             reading the box.
-  /// @return New box reader if successful, NULL otherwise.
+  /// @return New box reader if successful, NULL otherwise. Note that this
+  ///         function may return NULL if an intact, complete box is not
+  ///         available in the buffer. For MDAT box only, a BoxReader object is
+  ///         returned as long as the box header is available.
   static BoxReader* ReadTopLevelBox(const uint8* buf,
                                     const int buf_size,
                                     bool* err);
diff --git a/media/mp4/mp4_media_parser.cc b/media/mp4/mp4_media_parser.cc
index 25d9cf48fd..03f38d6b5d 100644
--- a/media/mp4/mp4_media_parser.cc
+++ b/media/mp4/mp4_media_parser.cc
@@ -108,6 +108,17 @@ bool MP4MediaParser::ParseBox(bool* err) {
   if (reader.get() == NULL)
     return false;
 
+  if (reader->type() == FOURCC_MDAT) {
+    // The code ends up here only if a MOOV box is not yet seen.
+    DCHECK(!moov_);
+
+    NOTIMPLEMENTED() << " Files with MDAT before MOOV is not supported yet.";
+    *err = true;
+    // TODO: Change to return a status so upper level can take appropriate
+    // actions based on the returned status.
+    return false;
+  }
+
   // Set up mdat offset for ReadMDATsUntil().
   mdat_tail_ = queue_.head() + reader->size();