diff --git a/telethon/network/mtproto_sender.py b/telethon/network/mtproto_sender.py
index 84dac60c..35f065ab 100644
--- a/telethon/network/mtproto_sender.py
+++ b/telethon/network/mtproto_sender.py
@@ -39,16 +39,6 @@ class MtProtoSender:
         """Disconnects from the server"""
         self._transport.close()
 
-    def _generate_sequence(self, confirmed):
-        """Generates the next sequence number, based on whether it
-           was confirmed yet or not"""
-        if confirmed:
-            result = self.session.sequence * 2 + 1
-            self.session.sequence += 1
-            return result
-        else:
-            return self.session.sequence * 2
-
     # region Send and receive
 
     def send(self, request):
@@ -142,7 +132,9 @@ class MtProtoSender:
             plain_writer.write_long(self.session.salt, signed=False)
             plain_writer.write_long(self.session.id, signed=False)
             plain_writer.write_long(request.msg_id)
-            plain_writer.write_int(self._generate_sequence(request.confirmed))
+            plain_writer.write_int(
+                self.session.generate_sequence(request.confirmed))
+
             plain_writer.write_int(len(packet))
             plain_writer.write(packet)
 
diff --git a/telethon/tl/session.py b/telethon/tl/session.py
index 14140648..a451b9f1 100644
--- a/telethon/tl/session.py
+++ b/telethon/tl/session.py
@@ -3,6 +3,7 @@ import os
 import pickle
 import random
 import time
+from threading import Lock
 from base64 import b64encode, b64decode
 from os.path import isfile as file_exists
 
@@ -52,6 +53,16 @@ class Session:
             else:
                 return Session(session_user_id)
 
+    def generate_sequence(self, confirmed):
+        """Ported from JsonSession.generate_sequence"""
+        with Lock():
+            if confirmed:
+                result = self.sequence * 2 + 1
+                self.sequence += 1
+                return result
+            else:
+                return self.sequence * 2
+
     def get_new_msg_id(self):
         """Generates a new message ID based on the current time (in ms) since epoch"""
         # Refer to mtproto_plain_sender.py for the original method, this is a simple copy
@@ -92,11 +103,14 @@ class JsonSession:
         self.port = 443
         self.auth_key = None
         self.id = utils.generate_random_long(signed=False)
-        self.sequence = 0
+        self._sequence = 0
         self.salt = 0  # Unsigned long
         self.time_offset = 0
         self.last_message_id = 0  # Long
 
+        # Cross-thread safety
+        self._lock = Lock()
+
     def save(self):
         """Saves the current session object as session_user_id.session"""
         if self.session_user_id:
@@ -105,7 +119,7 @@ class JsonSession:
                     'id': self.id,
                     'port': self.port,
                     'salt': self.salt,
-                    'sequence': self.sequence,
+                    'sequence': self._sequence,
                     'time_offset': self.time_offset,
                     'server_address': self.server_address,
                     'auth_key_data':
@@ -139,7 +153,7 @@ class JsonSession:
                     result.id = data['id']
                     result.port = data['port']
                     result.salt = data['salt']
-                    result.sequence = data['sequence']
+                    result._sequence = data['sequence']
                     result.time_offset = data['time_offset']
                     result.server_address = data['server_address']
 
@@ -155,7 +169,7 @@ class JsonSession:
                 result.id = old.id
                 result.port = old.port
                 result.salt = old.salt
-                result.sequence = old.sequence
+                result._sequence = old.sequence
                 result.time_offset = old.time_offset
                 result.server_address = old.server_address
                 result.auth_key = old.auth_key
@@ -163,6 +177,21 @@ class JsonSession:
 
             return result
 
+    def generate_sequence(self, confirmed):
+        """Thread safe method to generates the next sequence number,
+           based on whether it was confirmed yet or not.
+
+           Note that if confirmed=True, the sequence number
+           will be increased by one too
+        """
+        with self._lock:
+            if confirmed:
+                result = self._sequence * 2 + 1
+                self._sequence += 1
+                return result
+            else:
+                return self._sequence * 2
+
     def get_new_msg_id(self):
         """Generates a new unique message ID based on the current
            time (in ms) since epoch"""