Internet Relay Chat, or IRC, is an Internet protocol for interactive, real-time text messaging or synchronous conferencing, commonly called online chat or Internet chat. The primary use of IRC is group communication in forums called channels, in which messages from one user are relayed to all the other users currently signed on to that channel.

IRC channels and users often have modes that are indicated by a single case-sensitive letter, which are set using the command MODE. Channel and user modes are distinct from each other, and the same letters often indicate different things between the two mode types – for example, the letter s is a standard user mode indicating the user receives server notices, while as a channel mode, s typically indicates a secret channel.

An open protocol that uses the Transmission Control Protocol (TCP) and optional Transport Layer Security (TLS), IRC was originally a plain text protocol. While IRC specified that characters were 8-bit, it did not define the character encoding the text was supposed to use, a fact that can create some issues when IRC users employing different platforms or different clients try to chat with each other.

Created in 1988 by Finnish programmer Jarkko Oikarinen, IRC was written to supplant MUT, or Multi-User Talk, on a Finnish bulletin board system (BBS) called OuluBox, and was inspired by a messaging system called Bitnet Relay, which was used on a pre-Internet network called BITNET, a cooperative intercollegiate network founded in 1981.