A compose key, available on some computer keyboards, is a special kind of modifier key designated to signal the software to interpret the following sequence of two (or more) keystrokes as a combination in order to produce a character not found directly on the keyboard. For example, striking Compose followed by A and then E may produce the ligature æ, whereas striking Compose followed by O and then C can yield © (circled C, the copyright symbol).

The compose key can be found on the LK201 family of keyboards from Digital Equipment Corporation and its successors. The key can also be found on keyboards from Sun Microsystems. However, any key on a keyboard can be configured to serve as a compose key, as long as the functionality is supported by the software in use.

The compose key is different from a typical modifier key (such as AltGr or ⌥ Option) in that, rather than being pressed and held while another key is struck, it is pressed and released before striking the keys to be modified. Typically, the text-entry system does not indicate that any keys have been pressed until the composed character appears after the final keystroke of a predefined sequence. Therefore, the compose key can also be described as a kind of generic dead key. What makes the compose key different from a dead key is that while each dead key can only be used to attach a specific diacritic to various base letters, the compose key can be used to produce any kind of a character supported by the character set and font in use. This way, an even wider variety of special characters can be made available on a keyboard with a limited number of keys.