Methods and apparatus for mobile device identification using TCP segment timestamps are disclosed. A request from a mobile device is received at a server. A TCP connection, initiated by the mobile device, is made between the server and the device, and a timestamp generated by the mobile device is retrieved from the options field of the TCP SYN segment. Other identifying information is received from the mobile device source, including an IP address, port, and HTTP headers. Based on the identifying information, the mobile device timestamp, and a current timestamp generated by the server, a likelihood is determined that the particular mobile device previously communicated with the server.