Thomas Mark Harmon (born September 2, 1951) is an American actor, writer, producer, television director and former football player. He is best known for playing the lead role of Leroy Jethro Gibbs on NCIS.

He has appeared in a wide variety of television roles since the early 1970s, including as Dr. Robert Caldwell on St. Elsewhere, Detective Dicky Cobb on Reasonable Doubts, and Dr. Jack McNeil on Chicago Hope. He also starred in such films as Summer School, Prince of Bel Air, Stealing Home, Wyatt Earp, Fear and Loathing in Las Vegas, Crossfire Trail, Local Boys, Freaky Friday, and Chasing Liberty.

Harmon played Secret Service special agent Simon Donovan in a four-episode story arc in The West Wing in 2002, receiving an Emmy Award nomination for the role. Harmon's character of NCIS Special Agent Leroy Jethro Gibbs was introduced in a guest starring role in two episodes of JAG. From 2003 to 2021, Harmon starred in the spinoff NCIS as the same character.