Berkeley sockets (or BSD sockets) is a computing library with an application programming interface (API) for internet sockets and Unix domain sockets, used for inter-process communication (IPC).

As the API has evolved with little modification from a de facto standard into part of the POSIX specification, POSIX sockets are basically Berkeley sockets.

Berkeley sockets originated with the 4.2BSD Unix operating system (released in 1983) as an API. Only in 1989, however, could UC Berkeley release versions of its operating system and networking library free from the licensing constraints of AT&T's proprietary Unix. This interface implementation is the original API of the Internet Protocol Suite (TCP/IP).

All modern operating systems now have some implementation of the Berkeley socket interface, as it became the standard interface for connecting to the Internet. Even the Winsock implementation for MS Windows, developed by unaffiliated developers, closely follows the Berkeley standard.

As the Berkeley socket API evolved over time, and ultimately into the POSIX socket API, certain functions were deprecated or even removed and replaced by others. The POSIX API is also designed to be reentrant. These features now set the classic BSD API from the POSIX API apart.