Camacho, 50, whose speed and flamboyance made him a top attraction in the 1980s and 1990s, is in a coma and unresponsive, said Dr. Ernesto Torres, director of the Rio Piedras Medical Center.
Camacho "is clinically brain dead," Torres said. "The tests showed a flat line, without any type of activity."
Doctors will meet with his mother, Maria Matias, and other relatives on Friday to discuss options, Torres said. They include removing him from life support and organ donation.
Two gunmen opened fire on Camacho and a friend, Adrian Mojica Moreno, 49, late on Tuesday as they sat in a car outside a liquor store in the San Juan suburb of Bayamon, Camacho's birthplace.
Camacho was hit in the jaw and the bullet fractured two vertebrae and lodged in his shoulder. Torres said the bullet damaged three of the four arteries that carried blood to the brain.
A police spokeswoman said Mojica Moreno was killed and the gunmen fled in a vehicle. Officers found nine small bags of cocaine in the driver's pockets and one open in the car.
No arrests have been made and the investigation is ongoing, she said.
Ismael Leandry, Camacho's former manager, called him "a great friend and a great boss." He blamed a crime wave sweeping Puerto Rico for the shooting.
Camacho "took a ride with a friend who apparently had some problems in the street and got caught up in the shooting. They were not going after Macho," he said.
Camacho, a left-handed fighter who grew up in New York's tough Spanish Harlem, had a record of 79-6-3 with 38 knockouts. His three-decade career featured fights with a 'who's who' of boxing and a flamboyant style that included entering the ring in an outfit based on the Puerto Rican flag.
Camacho won the WBC vacant junior lightweight title by beating Rafael Limon in 1983. He then moved up to the lightweight division and took the WBC crown in 1985 by defeating Jose Luis Ramirez in Las Vegas.
Both the WBC titles were vacated after Camacho won them.
Camacho beat Ray 'Boom Boom' Mancini to take the vacant WBO light welterweight title in 1989. He is one of a handful of boxers to win championships in three weight classes, and the first to win in seven divisions.
Camacho lost the title to Greg Haugen in February 1991, then regained it in a rematch two months later. The title was vacated in 1992.
Camacho beat 'Sugar' Ray Leonard in 1997 and twice defeated Roberto 'Fists of Stone' Duran. His last world title fight was in 1997 and his last fight came in 2010.
Camacho battled drug and alcohol problems during his career. He was sentenced to seven years in prison in 2007 for the burglary of a Mississippi computer store. During the arrest, he was found to be in possession of the drug ecstasy.
All but one year of the sentence was suspended and he was given probation. He violated probation, however, and served two weeks in jail.