diff --git a/demo/common/assets.js b/demo/common/assets.js
index fb01c1a10b..3c563a5901 100644
--- a/demo/common/assets.js
+++ b/demo/common/assets.js
@@ -1406,7 +1406,8 @@ shakaAssets.testAssets = [
       /* source= */ shakaAssets.Source.MICROSOFT)
       .addFeature(shakaAssets.Feature.MSS)
       .addFeature(shakaAssets.Feature.HIGH_DEFINITION)
-      .addFeature(shakaAssets.Feature.MP4),
+      .addFeature(shakaAssets.Feature.MP4)
+      .addFeature(shakaAssets.Feature.OFFLINE),
   new ShakaDemoAssetInfo(
       /* name= */ 'Super Speedway Trailer (MSS - PlayReady)',
       /* iconUri= */ 'https://reference.dashif.org/dash.js/latest/samples/lib/img/mss-1.jpg',
diff --git a/externs/shaka/offline.js b/externs/shaka/offline.js
index 0d2696cc79..9971b25521 100644
--- a/externs/shaka/offline.js
+++ b/externs/shaka/offline.js
@@ -141,6 +141,7 @@ shaka.extern.ManifestDB;
  *   spatialAudio: boolean,
  *   closedCaptions: Map.<string, string>,
  *   tilesLayout: (string|undefined),
+ *   mssPrivateData: (shaka.extern.MssPrivateData|undefined),
  *   external: boolean,
  *   fastSwitching: boolean,
  *   isAudioMuxedInVideo: boolean
@@ -214,6 +215,9 @@ shaka.extern.ManifestDB;
  *   The value is a grid-item-dimension consisting of two positive decimal
  *   integers in the format: column-x-row ('4x3'). It describes the arrangement
  *   of Images in a Grid. The minimum valid LAYOUT is '1x1'.
+ * @property {(shaka.extern.MssPrivateData|undefined)} mssPrivateData
+ *   <i>Microsoft Smooth Streaming only.</i> <br>
+ *   Private MSS data that is necessary to be able to do transmuxing.
  * @property {boolean} external
  *   Indicate if the stream was added externally.
  *   Eg: external text tracks.
diff --git a/lib/offline/download_info.js b/lib/offline/download_info.js
index 45073787b7..62831f5329 100644
--- a/lib/offline/download_info.js
+++ b/lib/offline/download_info.js
@@ -7,6 +7,7 @@
 goog.provide('shaka.offline.DownloadInfo');
 
 goog.require('shaka.util.Networking');
+goog.require('shaka.util.Uint8ArrayUtils');
 goog.requireType('shaka.media.InitSegmentReference');
 goog.requireType('shaka.media.SegmentReference');
 
@@ -43,6 +44,10 @@ shaka.offline.DownloadInfo = class {
    * @return {string}
    */
   static idForSegmentRef(ref) {
+    const segmentData = ref.getSegmentData();
+    if (segmentData) {
+      return shaka.util.Uint8ArrayUtils.toBase64(segmentData);
+    }
     // Escape the URIs using encodeURI, to make sure that a weirdly formed URI
     // cannot cause two unrelated refs to be considered equivalent.
     const removeSprites = (uri) => {
diff --git a/lib/offline/download_manager.js b/lib/offline/download_manager.js
index b5e3390c1b..d6a6ffc7db 100644
--- a/lib/offline/download_manager.js
+++ b/lib/offline/download_manager.js
@@ -168,6 +168,60 @@ shaka.offline.DownloadManager = class {
     return newPromise;
   }
 
+  /**
+   * Add already-downloaded data to a group.
+   *
+   * @param {number} groupId
+   *    The group to add this segment to. If the group does not exist, a new
+   *    group will be created.
+   * @param {!BufferSource} queueData
+   * @param {number} estimateId
+   * @param {boolean} isInitSegment
+   * @param {function(BufferSource):!Promise} onDownloaded
+   *   The callback for when this request has been downloaded. Downloading for
+   *   |group| will pause until the promise returned by |onDownloaded| resolves.
+   * @return {!Promise} Resolved when this request is complete.
+   */
+  queueData(groupId, queueData, estimateId, isInitSegment, onDownloaded) {
+    this.destroyer_.ensureNotDestroyed();
+
+    const group = this.groups_.get(groupId) || Promise.resolve();
+
+    // Add another download to the group.
+    const newPromise = group.then(() => {
+      // Make sure we stop downloading if we have been destroyed.
+      if (this.destroyer_.destroyed()) {
+        throw new shaka.util.Error(
+            shaka.util.Error.Severity.CRITICAL,
+            shaka.util.Error.Category.STORAGE,
+            shaka.util.Error.Code.OPERATION_ABORTED);
+      }
+
+      // Update initData
+      if (isInitSegment) {
+        const segmentBytes = shaka.util.BufferUtils.toUint8(queueData);
+        const pssh = new shaka.util.Pssh(segmentBytes);
+        for (const key in pssh.data) {
+          const index = Number(key);
+          const data = pssh.data[index];
+          const systemId = pssh.systemIds[index];
+          this.onInitData_(data, systemId);
+        }
+      }
+
+      // Update all our internal stats.
+      this.estimator_.close(estimateId, queueData.byteLength);
+      this.onProgress_(
+          this.estimator_.getEstimatedProgress(),
+          this.estimator_.getTotalDownloaded());
+
+      return onDownloaded(queueData);
+    });
+
+    this.groups_.set(groupId, newPromise);
+    return newPromise;
+  }
+
   /**
    * Add additional async work to the group work queue.
    *
diff --git a/lib/offline/manifest_converter.js b/lib/offline/manifest_converter.js
index b07c6e9cc7..c88bfd4d1e 100644
--- a/lib/offline/manifest_converter.js
+++ b/lib/offline/manifest_converter.js
@@ -215,6 +215,7 @@ shaka.offline.ManifestConverter = class {
       spatialAudio: streamDB.spatialAudio,
       closedCaptions: streamDB.closedCaptions,
       tilesLayout: streamDB.tilesLayout,
+      mssPrivateData: streamDB.mssPrivateData,
       accessibilityPurpose: null,
       external: streamDB.external,
       fastSwitching: streamDB.fastSwitching,
diff --git a/lib/offline/storage.js b/lib/offline/storage.js
index 32ee8b2da6..2a208b46a9 100644
--- a/lib/offline/storage.js
+++ b/lib/offline/storage.js
@@ -511,8 +511,16 @@ shaka.offline.Storage = class {
           pendingDataSize += data.byteLength;
         };
 
-        downloader.queue(download.groupId,
-            request, estimateId, isInitSegment, onDownloaded);
+        const ref = /** @type {!shaka.media.SegmentReference} */ (
+          download.ref);
+        const segmentData = ref.getSegmentData();
+        if (segmentData) {
+          downloader.queueData(download.groupId,
+              segmentData, estimateId, isInitSegment, onDownloaded);
+        } else {
+          downloader.queue(download.groupId,
+              request, estimateId, isInitSegment, onDownloaded);
+        }
       }
       await downloader.waitToFinish();
 
@@ -1606,6 +1614,7 @@ shaka.offline.Storage = class {
       spatialAudio: stream.spatialAudio,
       closedCaptions: stream.closedCaptions,
       tilesLayout: stream.tilesLayout,
+      mssPrivateData: stream.mssPrivateData,
       external: stream.external,
       fastSwitching: stream.fastSwitching,
       isAudioMuxedInVideo: stream.isAudioMuxedInVideo,
diff --git a/test/offline/manifest_convert_unit.js b/test/offline/manifest_convert_unit.js
index a85be1974a..75ee311bcd 100644
--- a/test/offline/manifest_convert_unit.js
+++ b/test/offline/manifest_convert_unit.js
@@ -532,6 +532,7 @@ describe('ManifestConverter', () => {
       spatialAudio: false,
       closedCaptions: null,
       tilesLayout: undefined,
+      mssPrivateData: undefined,
       accessibilityPurpose: null,
       external: false,
       fastSwitching: false,
@@ -587,6 +588,7 @@ describe('ManifestConverter', () => {
       spatialAudio: streamDb.spatialAudio,
       closedCaptions: streamDb.closedCaptions,
       tilesLayout: streamDb.tilesLayout,
+      mssPrivateData: streamDb.mssPrivateData,
       accessibilityPurpose: null,
       external: streamDb.external,
       fastSwitching: streamDb.fastSwitching,