This article is about the telecommunication concept. "Hotline" is also the name of a pioneering peer-to-peer software suite developed by Hotline Communications.

In telecommunication, a hotline (also called an automatic signaling service or off-hook service) is a point-to-point communications link in which a call is automatically originated to the preselected destination without any additional action by the user when the end instrument goes off-hook.

Hotlines cannot be used to originate calls other than to preselected destinations. Various priority services that require dialing are not properly termed "hotlines."

Perhaps the most famous hotline was the so-called "red telephone" that linked the White House with the Kremlin during the Cold War. It was established in 1963 after the Cuban missile crisis made it clear that reliable, direct communications between the two great nuclear powers was a vital necessity (it was not actually a telephone, but a set of high-speed teleprinters). During the crisis, it took nearly 12 hours to receive and decode Nikita Khruschev's 3,000 word initial settlement message—a dangerously long time in the chronology of nuclear brinksmanship. By the time the U.S. had drafted a reply, a tougher message from Moscow had been received demaning that U.S. missiles be removed from Turkey; White House advisors at the time thought that the crisis could have been more quickly, and more easily, averted if communication had been faster. It was rumoured that this link was encrypted using the theoretically unbreakable one-time pad system. The hotline was used for the first time during the 1967 Arab-Israeli war when both superpowers informed each other of military moves which might have been provocative or ambiguous.

Derived from Federal Standard 1037C.