Detective Brian Cassidy is a fictional character played by Dean Winters in the American crime drama television series Law & Order: Special Victims Unit on NBC. A recurring cast member during the first season, Cassidy is a young and inexperienced detective with the New York Police Department's Special Victims Unit, and the original partner of John Munch (Richard Belzer).

The stress of the Special Victims Unit is too much for Cassidy, and he transfers to the Narcotics unit in the middle of the season. Cassidy later returns in the 13th-season finale working undercover for a pimp the SVU detectives are investigating.

Winters had previously worked with creator Dick Wolf, but he was offered a role on SVU through Belzer, though the role proved to be temporary due to Winters' commitment to Oz. As Winters only remained with the series for 13 episodes, his character never had much of a chance to develop. Winters wanted to return to the show for a long time, however, and felt the time was right during the 13th-season finale. Critics were generally negative about Cassidy during the character's initial tenure on the show, but some were glad to see his return in "Rhodium Nights".

Even though Brian Cassidy is a dedicated member of the Special Victims Unit, he lacks the language to describe sex crimes and the emotional maturity to deal with them. He views his partner, John Munch (Richard Belzer), as a mentor.

Cassidy has a drunken one-night stand with fellow SVU Detective Olivia Benson (Mariska Hargitay) but wants to pursue a relationship with her. She turns him down, citing a policy of not having relationships with co-workers. Years later, Benson expresses regret at the way she handled the situation.

These factors and the stress of the unit wear Cassidy down. After Cassidy loses his temper at Benson in front of the squad, Captain Donald Cragen (Dann Florek) tests his readiness to deal with sex crimes by sending him to check up on a teenage rape survivor Cragen met when he worked in homicide. After she details how her abuser systematically violated her and then had her gang-raped, Cassidy decides he lacks the stomach to deal with sex crimes and transfers to the NYPD's Narcotics Division. Munch later laments that he felt abandoned when Cassidy became the latest in a long line of partners who left him.

Besides a brief appearance whilst undercover as a bartender in Season 9 episode "Authority", Cassidy makes a proper return 12 years later in the Season 13 episode "Rhodium Nights". He has been working undercover for three years as a bodyguard for a pimp, Bart Ganzel (Peter Jacobson), and gives the SVU detectives information about Ganzel's prostitution ring. When a prostitute who had been dating both Cassidy and Ganzel is found dead in bed with Cragen, Detective Nick Amaro (Danny Pino) believes Cassidy played a role in framing Cragen, despite the other SVU detectives vouching for him. Amaro confronts Cassidy at gunpoint and tries to force information out of him. After Ganzel discovers Cassidy is working undercover to expose him, he has a corrupt NYPD police officer shoot Cassidy. Amaro helps uncover the conspiracy that led to the attack. Cassidy survives and awakens in the hospital, where Benson tells him she is not the same person she was when they first worked together, and kisses him.

In the episode "Undercover Blue", a prostitute accuses Cassidy of raping her while he was undercover four years prior. Investigation reveals the woman and her boss set Cassidy up to make money from suing the NYPD, and the charges are dropped. Because of the longtime consensual relationship Cassidy had had with a prostitute during his undercover stint, however, he is demoted from detective to officer, working nights at a Bronx courthouse. Benson's and Cassidy's relationship, until now a secret to SVU, is also forced into the open in this episode when Amaro and Munch go to Cassidy's apartment and find Benson there; in Season 15, the couple moves in together.