In computer software, specifically networking, the Transport Layer Interface (TLI) was the networking API provided by AT&T UNIX System V Release 3.0, and was the System V counterpart to BSD sockets. TLI was later standardised as XTI , the X/Open Transport Interface.

Although TLI and STREAMS were first introduced with SVR3, no actual protocol implementations were provided until SVR4 shipped with TCP/IP support. It was expected at the time that the OSI protocols would supersede TCP/IP, and TLI is designed from an OSI model-oriented viewpoint, corresponding to the OSI transport layer.

TLI and XTI were never as widely used as BSD sockets, and although they are still supported in SVR4-derived operating systems such as Solaris, sockets are now the de-facto standard networking API.