OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD. According to de Raadt, OpenBSD is a research operating system for developing security mitigations.

The OpenBSD project maintains portable versions of many subsystems as packages for other operating systems. Because of the project's emphasis on code quality, many components are reused in other software projects. Android uses a lot of its C standard library, LLVM uses its regular expression library, and Windows 10 uses OpenSSH (aka OpenBSD Secure Shell) with LibreSSL.

The word "open" in the name OpenBSD refers to the availability of the operating system's source code on the Internet, although the word "open" in the name OpenSSH means "OpenBSD". It also refers to the wide range of hardware platforms the system supports.

The OpenBSD project produces a FREE, multi-platform 4.4 BSD-based UNIX-like operating system. Our efforts emphasize portability, standardization, correctness, proactive security and integrated cryptography. As an example of the effect OpenBSD has, the popular OpenSSH software comes from OpenBSD.

The current release is OpenBSD 7.5, released April 5, 2024. This is the 56th release.

OpenBSD is developed entirely by volunteers. The project's development environment and developer events are funded through contributions collected by The OpenBSD Foundation. Contributions ensure that OpenBSD will remain a vibrant and free operating system.