Telnet (teletype network) is a network protocol used on the Internet or local area networks to provide a bidirectional interactive communications facility. Typically, telnet provides access to a command-line interface on a remote host via a virtual terminal connection which consists of an 8-bit byte oriented data connection over the Transmission Control Protocol (TCP). User data is interspersed in-band with TELNET control information.

Telnet was developed in 1969 beginning with RFC 15, extended in RFC 854, and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards.

The term telnet may also refer to the software that implements the client part of the protocol. Telnet client applications are available for virtually all computer platforms. Most network equipment and operating system with a TCP/IP stack support a Telnet service for remote configuration (including systems based on Windows NT). Because of security issues with Telnet, its use has waned in favor of SSH for remote access.

Telnet is also used as a verb. To telnet means to establish a connection with the Telnet protocol, either with command line client or with a programmatic interface. For example, a common directive might be: "To change your password, telnet to the server, login and run the passwd command." Most often, a user will be telnetting to a Unix-like server system or a network device such as a router and obtain a login prompt to a command line text interface or a character-based full-screen manager.

On many systems, a Telnet client application may also be used to establish interactive raw-TCP sessions. It is commonly believed that a Telnet session which does not use the IAC (character 255) is functionally identical. This is not the case, however, because there are other network virtual terminal (NVT) rules, such as the requirement for a bare carriage return character (CR, ASCII 13) to be followed by a NULL (ASCII 0) character, that distinguish the telnet protocol from raw-TCP sessions.