NCIS: Los Angeles is an American military police procedural television series that aired on CBS from September 22, 2009, to May 21, 2023. Created by Shane Brennan, the series starred Chris O'Donnell, Daniela Ruah, and LL Cool J with an ensemble cast, as it follows the exploits of the LA-based Office of Special Projects (OSP), an elite division of the Naval Criminal Investigative Service (NCIS) that specializes in undercover assignments. NCIS: Los Angeles is the first spin-off of the successful series NCIS and the second series in the NCIS franchise.

The first NCIS spin-off was announced in 2008 and was introduced via two-part back-door pilot episodes in season six of NCIS. Louise Lombard, who played Lara Macy in the pilot episodes, was not included in the main series. The cast of the first season featured Peter Cambor, Adam Jamal Craig, Linda Hunt and Barrett Foa. Cambor was demoted to recurring status, and Craig's character was killed off at the end of season 1, while Foa was written out at the end of season 12 and Hunt was demoted to "special guest star" status at the beginning of season 13. Other stars in other seasons included Eric Christian Olsen, Renée Felice Smith, Miguel Ferrer, Nia Long, Medalion Rahimi, Caleb Castille, and Gerald McRaney. Cast members from the original NCIS made appearances on the series with Rocky Carroll recurring in the first three seasons, and making an appearance in season six. Cast members from non-NCIS shows had also made appearances, specifically in a crossover event with Hawaii Five-0.

NCIS: Los Angeles received generally mixed-to-positive reviews from critics, but became a solid ratings hit for CBS at the time of its premiere. The series received six Teen Choice Awards for Choice Action Show, Choice TV Actress Action for Hunt, and Choice TV Actor Action for LL Cool J.

NCIS: Los Angeles follows Special Agents Sam Hanna (LL Cool J) and Grisha Callen (Chris O'Donnell), undercover agents assigned to the Office of Special Projects, a special branch of the Naval Criminal Investigative Service that specializes in undercover assignments. Sam is an ex-US Navy SEAL and former member of SEAL Team Six and dedicated family man. Callen is a former foster child who became a "legend" under the watchful eye of Operations Manager Henrietta "Hetty" Lange (Linda Hunt). At the start of the series, Sam, Callen, and Hetty are supported by Special Agent Kensi Blye (Daniela Ruah), a hand-to-hand combat specialist, trained sniper and forensic whiz, rookie agent Dominic Vail (Adam Jamal Craig), Operational Psychologist Nate Getz (Peter Cambor) and Technical Operator Eric Beale (Barrett Foa). Vail is abducted by terrorists halfway through season 1 and, following a period of being held hostage, is killed during his rescue mission near the end of the season. Getz, meanwhile, enters training to become an agent in the back-half of the season, and is reassigned during season 2, returning for occasional guest appearances afterwards.

In season 2, LAPD Detective Marty Deeks (Eric Christian Olsen), after helping on several cases late in the prior season, joins the OSP team as a liaison between LAPD and NCIS, replacing Vail as Kensi's partner. He holds this position until departmental reforms in season 12 end up terminating it, after which he completes FLETC training and officially joins NCIS as an Investigator. Over the course of the series, Deeks and Kensi slowly evolve from partners to lovers, with the two marrying in season 10. The two then become foster parents to immigrant teenager Rosa in season 13 and are revealed to be expecting their own baby in the series finale. Also joining the team in season 2 is Intelligence Analyst Nell Jones (Renee Felice Smith), a brilliant agent whom Hetty hopes to have take her place eventually, and who forms a close relationship with Beale.

During season 3, Assistant Director Owen Granger (Miguel Ferrer), an old friend of Hetty's, is assigned to the team as oversight with her, and manages to develop a cordial relationship with the team despite a frosty start. In season 8, Granger falls ill with cancer, coinciding with Ferrer's real-life battle with the disease; following Ferrer's death, Granger is written to have fled the hospital while in recovery from a failed assassination attempt, and is later confirmed to have died from his illness during the following season.

In season 9, Executive Assistant Director Pacific (EADPAC) Shay Mosley (Nia Long) is assigned to the OSP after Hetty leaves on a personal mission to free an old colleague in Vietnam; the team later rescues her when the mission goes wrong. Mosley, meanwhile, sends the team on an unsanctioned mission to Mexico to rescue her son from her criminal ex-husband at the end of the season; while successful in its goals, the mission results in the death of Mosley's trusted assistant, the wounding of most of the rest of team, and the OSP being put under the microscope during the beginning of season 10; Mosley later accepts full responsibility, resigns and goes into hiding with her son to escape a cartel allied with her ex (whom she had killed during the mission).

During seasons 11 and 12, rookie agents Fatima Nazami (Medalion Rahimi) and Devin Rountree (Caleb Castille) are recruited to the OSP, while Beale and Jones depart for the private sector at the end of the latter season. With Hetty leaving to do damage control from a prior mission, Admiral Hollace Kilbride (Gerald McRaney), who has assisted the OSP in multiple cases, is assigned in her place as Director of Special Operations in season 13. The series ends with Callen marrying long-time love Anna Kolcheck, and him and Sam taking a "side project" mission to find and rescue Hetty with a new team, which includes old teammates Nate and Nell.