SSN is a three-letter abbreviation with multiple meanings:

• Social Security Number, used by Social Security (United States).
• SSN, the hull classification symbol used by the United States Navy for an attack submarine that is propelled by nuclear energy, such as the Los Angeles-class of attack submarines.
• A 1996 novel by Tom Clancy describing the operations of a US Navy attack submarine
• Socialist Solidarity Network, a Trotskyist group in the UK.
• Sub System Number used in SCCP, SS7 routing

See also: SS, SSBN, Submarine, Global_Title_Translation