Psi is a free instant messaging client for the XMPP protocol which uses the Qt toolkit. It runs on Linux (and other Unix-like operating systems), Windows, macOS and OS/2 (including eComStation and ArcaOS).

User interface of program is very flexible in customization. For example, there are "multi windows" and "all in one" modes, support of different iconsets and themes.

Ready-to-install deb and RPM packages are available for many Linux distributions. Successful ports of Psi were reported for Haiku, FreeBSD and Sun Solaris operating systems.

Due to Psi's free/open-source nature, several forks have appeared, which occasionally contain features that may appear in future official Psi versions.

'Psi' is the twenty-third letter of the Greek alphabet (Ψ), which is used as the software's logo.

The goal of the Psi project is to create a powerful, yet easy-to-use XMPP client that tries to strictly adhere to the XMPP drafts and XMPP XEPs. This means that in most cases, Psi will not implement a feature unless there is an accepted standard for it in the XMPP community. Doing so ensures that Psi will be compatible, stable, and predictable.

The application was created by Justin Karneges and it began as a side project. At various points during its existence Karneges was paid to develop the codebase, during which Psi flourished. Typically however, the release cycle of Psi is relatively slow, but the client has always been seen by its fans as a very stable and powerful instant messaging client. Karneges left the project in late 2004 to pursue other endeavors. In 2002 Michail Pishchagin started hacking Qt code which later became libpsi library. Pishchagin joined the team in March 2003 and he is responsible for many large chunks in Psi code.

In November 2004, maintenance was taken over by Kevin Smith, a long-time contributor to the project. In 2009, Smith handed maintenance back to Karneges, who also maintains Iris, the Qt/C++ XMPP library upon which Psi is based.