John James McCoy is a fictional character in the American television drama Law & Order. He was created by Dick Wolf and Michael S. Chernuchin and has been portrayed by Sam Waterston during the show's original run from 1994 to 2010 and again from 2022 until his retirement in 2024. He is the longest-tenured character on the show, appearing in 19 seasons. In addition to appearances in 405 episodes of Law & Order, the character appeared in other shows across the Law & Order franchise, including four episodes of Law & Order: Special Victims Unit, two episodes of Law & Order: Trial by Jury, two crossover episodes of Homicide: Life on the Street, and the made-for-TV movie Exiled.

Waterston's performance as McCoy on the New York–based series was so popular that it resulted in Waterston being declared a "Living Landmark" by the New York Landmarks Conservancy, in a joint recognition with fellow longtime series cast member Jerry Orbach, who portrayed NYPD detective Lennie Briscoe on Law & Order for 12 seasons.

McCoy is introduced as Executive Assistant District Attorney by Adam Schiff (Steven Hill) in the season five premiere episode "Second Opinion". He has been nicknamed "Hang 'em High McCoy". He describes himself as a "junkyard dog". He is well-respected in the legal community, once referred to as "the top of the legal food chain" by a rival attorney during a trial.

Following the 17th season (2006–2007), McCoy is appointed interim district attorney, taking over from Arthur Branch (Fred Thompson). McCoy's appearance on Law & Order: Special Victims Unit on the November 13, 2007, episode "Blinded" marked his first appearance in the Law & Order universe as district attorney.

Law & Order originally ceased production in 2010, but McCoy (though not seen) was still occasionally referenced as the Manhattan DA in Law & Order: Special Victims Unit in several episodes through 2011. A reference to "the new DA" in a 2013 SVU episode suggested that McCoy had left the position, although his replacement was unnamed. Among the new DA's staffing changes were making Cutter a Bureau Chief ADA of the Special Victims Unit. Cutter, in turn, was replaced as EADA by David Haden. The new DA's career was eventually one of several ended by the massive statewide scandal involving madam Delia Wilson and her sex trafficking ring.

This made McCoy the DA again in 2018, appearing in the SVU episode "The Undiscovered Country". In 2022, he appeared as the DA in the revival of the original Law & Order, with ADAs Nolan Price (Hugh Dancy) and Samantha Maroun (Odelya Halevi) working under him.

Waterston made his final appearance as McCoy in the 2024 episode "Last Dance". In the episode, a case in which a politically connected tech billionaire is charged with murdering a woman he had raped years earlier earns McCoy the enmity of New York City Mayor Robert Payne (Bruce Altman), who threatens to destroy his career and those of Nolan and Maroun unless they back off from calling his son as a witness, which would reveal the younger Payne's extramarital affairs. McCoy prosecutes the case himself and obtains a guilty verdict; he then resigns as DA so the governor will appoint an interim successor, thus denying the vindictive Payne the chance to run him out of office and replace him with a lackey who would fire every ADA on his staff.

McCoy was abused by his father, an Irish Chicago policeman who had also beat Jack's mother, and who eventually died of cancer. McCoy says that his determination and unyielding work ethic are a byproduct of having been harshly punished by his father for losing at anything. He also revealed that his father was a racist who once hit him for dating a Polish girl. McCoy disliked his father, calling him a "son-of-a-bitch"; however, he admits he could have easily become like him.