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Keychain is Apple Computer's password management system in Mac OS X and Mac OS 9. It was introduced with Mac OS 8.6. A Keychain can contain various types of data: passwords (Websites, FTP servers, network shares, wireless networks, groupware applications, encrypted disk images), private keys, certificates and secure notes. The default keychain file is the login keychain, decrypted on login by the user's login password (this can be changed). Keychain files are stored in ~/Library/Keychains/.

Keychains were initially developed for Apple's e-mail system, PowerTalk. Among its many features, PowerTalk incorporated a powerful encryption system for security and digital signatures. The keychain concept naturally "fell out" of this code, and was used in PowerTalk to manage all of a user's various login credentials for the various e-mail systems PowerTalk could connect to. Keychain placed these passwords in an encrypted file, and automatically returned them on command if the file was "opened" using a password.

This offered excellent security not found on other platforms; the passwords were not easily retrievable due to the encryption, yet the simplicity of the interface allowed the user to select a different password for every system without fear of forgetting them, as a single password would open the file and return them all. At the time, this was a truly innovative concept that was not available on other platforms. Keychain was one of the few parts of PowerTalk that was obviously useful "on its own", which suggested it should be promoted to become a part of the basic MacOS. But due to internal politics, it was kept inside the PowerTalk system, and therefore available to very few Mac users.

It was not until the return of Steve Jobs that Keychain was liberated from the now-dead PowerTalk. By this point in time the concept was no longer so unique, but it was still rare to see a Keychain system that was not associated with a particular piece of software, typically a web browser. Keychain became a standard part of OS 9, and was included in OS X in the first commercial versions.

Third party uptake of Keychain has been somewhat spotty to date. Although most Apple software uses it (notably Apple Mail and Safari), and Macintosh-only applications such as Transmit and Camino do as well, cross-platform applications such as Firefox do not use Keychain, sticking to custom cross-platform solutions instead. Many programs continue to store their login credentials in plain text files, although this is becoming rare for newer programs.

Keychain Access