John Vincent Martorano (born December 13, 1940), also known as "Vincent Joseph Rancourt", "Richard Aucoin", "Nick", "The Cook", "The Executioner", "The Basin Street Butcher", is an American former mobster and hitman for the Winter Hill Gang in Boston, Massachusetts, who has admitted to 20 mob-related killings.

John Martorano was born in Somerville, Massachusetts, in 1940. He is the older brother of James "Jimmie" Martorano by eleven months. His father, Angelo "Andy" Martorano was an immigrant from Riesi, Sicily, and with his family he emigrated to the United States around 1915, to East Boston. His mother, Elizabeth Mary "Bess" Hunt, was of English and Irish descent, who lived in Somerville. Martorano was raised Catholic and served as an altar boy.

The Martorano family moved to the Irish enclave of East Milton. Martorano and his brother attended St Agatha's parochial grammar school in Milton through grade 8, where Martorano was a classmate of future congressman Bill Delahunt. Martorano attended Mount Saint Charles Academy in Woonsocket, Rhode Island as a freshman while his brother remained in Milton, enrolling in Cunningham Junior High School. Later, in his freshman year, Martorano dropped out of Mount Saint Charles and joined Jimmie at Cunningham. During high school, he and Jimmie were standout football players, and were elected co-captains of the team for their senior season in 1958. Although recruited by several college teams, Martorano did not continue his education beyond his graduation from Milton High.

In a 60 Minutes interview with Steve Kroft, Martorano claimed that when he was young his father told him, "You're the oldest son and this is your heritage" (referring to his father's connections to organized crime). "You've got to take care of your family and be a man."

After graduating from high school, Martorano turned down seven football scholarships and instead stayed in Boston. Hanging out in the Combat Zone, Martorano fell under the guidance of Stephen Flemmi, and by the age of 25 was an active mobster. He committed his first murder at 24, when he allegedly killed Patriarca crime family made man Robert S. Palladino, who was going to testify in a case involving the murder of prostitute Barbara Sylvester in his father's restaurant.

Ralph DeMasi, a Boston mobster incarcerated in White Deer Township, Pennsylvania, later wrote to the courts that when he was driving down Morrissey Boulevard with fellow Irish mobster William (Billy) O'Brien in 1964, Martorano pulled up in a car alongside them and gunned down O'Brien, shooting him seventeen times with a machine gun and wounding DeMasi. In his letter about the events that almost led to his death he wrote, "I thought someone was taking target practice at us. It was my good friend John Martorano."

Martorano rapidly became one of the Winter Hill Gang's most prolific enforcers under the tenures of Howie Winter and Whitey Bulger. In January 1968, after Hubert "Smitty" Smith, an African-American man, helped mobsters Rocco Lammattina and John Cincotti beat up Flemmi in an after-hours saloon, Martorano confronted Smith at the saloon the next night. Despite his ignorance of the circumstances behind Flemmi's beating, Martorano challenged Smith and questioned him about the altercation between Lammattina and Cincotti versus Flemmi. When recounting Smith's responses, Martorano said, "He (Smith) kept giving me the wrong answers. He didn't give me any respect. All he had to say was 'I didn't know he was your friend, I'm sorry.' That's all he needed to say." After Smith did not answer Martorano's challenge satisfactorily, Martorano tracked Smith to a car on Normandy Street in Dorchester. Smith was accompanied by a 19-year-old woman, Elizabeth Frances "Liz" Dickson, and a 17-year-old boy, Douglas Barrett. Martorano walked up to the car and killed all three occupants with his .38-caliber snubnosed revolver.

On March 8, 1973, Martorano machine gunned 30-year-old bartender Michael Milano to death as he was driving in the Brighton neighborhood of Boston after mistaking Milano for a rival gang leader, Al Notarangeli. Milano drove a Mercedes-Benz similar to Notarangeli's and wore a fur coat like the one Notarangeli wore. Milano's friend, Louis Lapiana, and Lapiana's girlfriend, Dianne Sussman, were wounded in the shooting. Lapiana was left paralyzed until his death in 2001. The Winter Hill Gang held a fundraiser for Lapiana at Chandler's bar after the shooting without disclosing to him that it was them who had shot him.