The Federal Bureau of Investigation (FBI) is a Federal Agency based and located in the United States. Its job is to investigate possible terrorist offences or crimes related to terrorism, murder or drugs.
- Charles- Was originally Fornell's partner, Charles first appeared in Enigma (episode) but later became a mole for a Mafia crime boss, Jimmy Napolitano. Died in The Bone Yard (episode) after being shot by a man with a goatee who had a sniper rifle. His position was later taken over by Ron Sacks.
- Tobias Fornell- A FBI Agent and a friend to Leroy Jethro Gibbs. He has a daughter from his marriage to one of Gibbs's ex-wives, Diane Sterling. He regulary helps NCIS whenever their investigations fall into FBI terrority. Although they were cold to each other in Season 1, the two have since become good friends.
- Brent Langer- A former member of the FBI who joined Gibbs's team after new Director Leon Vance reassigned the original members of his team. Langer was later killed in cold-blood by mole Michelle Lee although she claimed self-defence for her actions.
- Maya- A FBI Agent who appeared in Under Covers (episode).
- Samantha Morris- A FBI Agent who appeared in Toxic (episode).
- Lina Reyes- An Agent who appeared in Terminal Leave (episode). Had something of a very frosty relationship with NCIS who she believed were only good to protect a Iraqi veteran and her family who were under threat from a group of terrorists while Reyes believed that the FBI should handle the plan of exposing and capturing the terrorist cell responsible for threats against the Commander even if it meant putting the Shields family at risk.
- Ron Sacks- An FBI agent who became Fornell's new partner after Agent Charles's death. Has something of a frosty relationship with Anthony DiNozzo.
- William Gamal- AKA "Bill", he's an FBI translator presumbly assigned to Gitmo who appeared in Minimum Security (episode).
- Yussif- Maya's partner who appeared in Under Covers.