The Full Form of NCIS is Naval Criminal Investigative Service.

The United States Naval Criminal Investigative Service (NCIS) is the primary law enforcement agency of the U.S. Department of the Navy. Its primary function is to investigate criminal activities involving the U.S. Navy and U.S. Marine Corps, though its broad mandate includes national security, counterintelligence, counter-terrorism, cyber warfare, and the protection of U.S. naval assets worldwide. NCIS is the successor organization to the former Naval Investigative Service (NIS), which was established by the Office of Naval Intelligence after the Second World War.

The vast majority of NCIS personnel are civilian, with half being special agents. NCIS agents are armed federal law enforcement investigators, who frequently coordinate with other U.S. government agencies and have a presence in more than 41 countries and on U.S. Navy vessels. NCIS special agents are supported by analysts and other experts skilled in disciplines such as forensics, surveillance, surveillance countermeasures, computer investigations, physical security, and polygraph examinations.