NCIS, also known as NCIS: Naval Criminal Investigative Service, is an American police procedural drama television series revolving around a fictional team of special agents from the Naval Criminal Investigative Service which conducts criminal investigations involving the U.S. Navy and Marine Corps.

The concept and characters were initially introduced in a two-part episode of the CBS series JAG (JAG episodes 8.20 and 8.21). The show, a spin-off from JAG, premiered on September 23, 2003 on CBS and, to date, has aired six full seasons and has gone into syndicated reruns on USA Network, Sleuth and Ion Television. Donald Bellisario, who created JAG as well as the well-known series and Quantum Leap, is co-creator and executive producer of NCIS.

NCIS was originally referred to as "Navy NCIS" during Season 1; however, "Navy" was later dropped from the title as it was redundant. NCIS returned for a seventh season on September 22, 2009 at 8:00 p.m. EST and was followed by its spin-off series NCIS: Los Angeles starring Chris O'Donnell and LL Cool J.