diff --git a/testing/marionette/client/marionette/marionette.py b/testing/marionette/client/marionette/marionette.py
index 48a764381b71..d1b71c61b142 100644
--- a/testing/marionette/client/marionette/marionette.py
+++ b/testing/marionette/client/marionette/marionette.py
@@ -479,7 +479,6 @@ class Marionette(object):
         self.bin = bin
         self.instance = None
         self.session = None
-        self.session_id = None
         self.window = None
         self.runner = None
         self.emulator = None
@@ -608,12 +607,12 @@ class Marionette(object):
 
     @do_crash_check
     def _send_message(self, command, response_key="ok", **kwargs):
-        if not self.session_id and command != "newSession":
+        if not self.session and command != "newSession":
             raise errors.MarionetteException("Please start a session")
 
         message = {"name": command}
-        if self.session_id:
-            message["sessionId"] = self.session_id
+        if self.session:
+            message["sessionId"] = self.session
         if kwargs:
             message["parameters"] = kwargs
 
@@ -638,8 +637,6 @@ class Marionette(object):
                 continue;
 
             break;
-        if not self.session_id:
-            self.session_id = response.get("sessionId", None)
 
         if response_key in response:
             return response[response_key]
@@ -814,7 +811,7 @@ class Marionette(object):
         '''
         return "%s%s" % (self.baseurl, relative_url)
 
-    def start_session(self, session_id=None, desired_capabilities=None):
+    def start_session(self, desired_capabilities=None):
         """Create a new Marionette session.
 
         This method must be called before performing any other action.
@@ -823,7 +820,7 @@ class Marionette(object):
             capabilities.  This is currently ignored.
 
         :returns: A dict of the capabilities offered."""
-        self.session = self._send_message('newSession', 'value', capabilities=desired_capabilities, session_id=session_id)
+        self.session = self._send_message('newSession', 'value', capabilities=desired_capabilities)
         self.b2g = 'b2g' in self.session
         return self.session
 
@@ -839,7 +836,6 @@ class Marionette(object):
     def delete_session(self):
         """Close the current session and disconnect from the server."""
         response = self._send_message('deleteSession', 'ok')
-        self.session_id = None
         self.session = None
         self.window = None
         self.client.close()
diff --git a/testing/marionette/client/marionette/tests/unit/test_session.py b/testing/marionette/client/marionette/tests/unit/test_session.py
index 6ef600e48821..16eec76d3a59 100644
--- a/testing/marionette/client/marionette/tests/unit/test_session.py
+++ b/testing/marionette/client/marionette/tests/unit/test_session.py
@@ -29,16 +29,4 @@ class TestSession(marionette_test.MarionetteTestCase):
         self.assertIn("takesScreenshot", caps)
         self.assertIn("version", caps)
 
-    def test_we_can_get_the_session_id(self):
-        # Sends newSession
-        caps = self.marionette.start_session()
 
-        self.assertTrue(self.marionette.session_id is not None)
-        self.assertTrue(isinstance(self.marionette.session_id, unicode))
-
-    def test_we_can_set_the_session_id(self):
-        # Sends newSession
-        caps = self.marionette.start_session(session_id="ILoveCheese")
-
-        self.assertEqual(self.marionette.session_id, "ILoveCheese")
-        self.assertTrue(isinstance(self.marionette.session_id, unicode))
\ No newline at end of file
diff --git a/testing/marionette/marionette-server.js b/testing/marionette/marionette-server.js
index 3279b7485bd6..810b7f3c7fdd 100644
--- a/testing/marionette/marionette-server.js
+++ b/testing/marionette/marionette-server.js
@@ -127,7 +127,6 @@ function MarionetteServerConnection(aPrefix, aTransport, aServer)
   // passing back "actor ids" with responses. unlike the debugger server,
   // we don't have multiple actors, so just use a dummy value of "0" here
   this.actorID = "0";
-  this.sessionId = null;
 
   this.globalMessageManager = Cc["@mozilla.org/globalmessagemanager;1"]
                              .getService(Ci.nsIMessageBroadcaster);
@@ -331,9 +330,7 @@ MarionetteServerConnection.prototype = {
   sendResponse: function MDA_sendResponse(value, command_id) {
     if (typeof(value) == 'undefined')
         value = null;
-    this.sendToClient({from:this.actorID,
-                       sessionId: this.sessionId,
-                       value: value}, command_id);
+    this.sendToClient({from:this.actorID, value: value}, command_id);
   },
 
   sayHello: function MDA_sayHello() {
@@ -556,8 +553,6 @@ MarionetteServerConnection.prototype = {
 
     this.scriptTimeout = 10000;
     if (aRequest && aRequest.parameters) {
-      this.sessionId = aRequest.parameters.session_id ? aRequest.parameters.session_id : null;
-      logger.info("Session Id is set to: " + this.sessionId);
       this.setSessionCapabilities(aRequest.parameters.capabilities);
     }
 
@@ -630,10 +625,6 @@ MarionetteServerConnection.prototype = {
   getSessionCapabilities: function MDA_getSessionCapabilities() {
     this.command_id = this.getCommandId();
 
-    if (!this.sessionId) {
-      this.sessionId = this.uuidGen.generateUUID().toString();
-    }
-
     // eideticker (bug 965297) and mochitest (bug 965304)
     // compatibility.  They only check for the presence of this
     // property and should so not be in caps if not on a B2G device.
@@ -2286,7 +2277,6 @@ MarionetteServerConnection.prototype = {
     if (this.mainFrame) {
       this.mainFrame.focus();
     }
-    this.sessionId = null;
     this.deleteFile('marionetteChromeScripts');
     this.deleteFile('marionetteContentScripts');
   },