Notorious drug lord-turned-federal witness Alberto “Alpo” Martinez was gunned down by a killer who opened fire on his truck on a Harlem street Sunday morning, police sources said.

Martinez, 55, was behind the wheel of a Dodge Ram on Frederick Douglass Blvd. near W. 152nd St. when several gunshots exploded through his driver’s side window just after 3:20 a.m., police sources said.

Word of the rubout sparked a celebration from the family of Rich Porter, Martinez’s business partner, and best friend who he notoriously shot to death in 1990.

A character based on Martinez was portrayed by the rapper Cam’ron in the 2002 film, “Paid in Full.” Mekhi Phifer played a character based on Porter, who was 24 when he was killed, and Jay-Z was among the movie’s producers.

Porter’s family was celebrating in the streets of Harlem with champagne Sunday.

“We waited for a long time for this day to come and we are happy. That’s why we’re out here celebrating, drinking champagne,” a 37-year-old niece of Porter’s who gave her name only as Lorell said as she poured herself more bubbly into a fancy glass. “Everybody’s reaction right now is celebrating. It’s a celebration for Harlem, period. Not even my family.”

Martinez’s truck rear-ended another motorist who told police he heard the shots but didn’t see what happened, sources said. The Dodge Ram then crashed into three parked cars by W. 147th St., sources said.

Word of his demise drew shocked neighbors to the scene throughout the day Sunday.

“Alpo finally got caught slipping,” one passerby said. “They shot him in the f---ing face. They finally got that rat.”

“I don’t want anyone to be hurt,” another neighborhood resident told the Daily News, declining to give her name. “But if you hear the whole situation with Alpo, it’s like, damn, now y’all getting to it.”

A group of about six people, believed to be his relatives, showed up at the scene Sunday morning.

“No, God, why!” one woman in the group said, as she collapsed onto the hood of a car, weeping.

“You motherf---ers happy now?” a man in the group yelled.

Police found Martinez gasping for air in the truck, shot six times in his arm and once in his chest, sources said. He also had a graze wound on the left side of his chin.

Medics rushed him to Harlem Hospital, but he couldn’t be saved. Cops have made no immediate arrests.

Martinez was a controversial figure on the streets of Harlem after he testified against underlings in his murderous drug-trafficking ring to avoid a life sentence — including his enforcer, Wayne Perry, who took a life sentence to avoid the death penalty in 1994, according to coverage from the Washington Post.

Martinez was released into a witness protection program in 2015 and was awaiting a new identity, Don Diva magazine reported. Even so, he surfaced in 2019 in Harlem, talking to a crowd of people on the street, a video posted on Instagram shows.

Police identified the victim Sunday as Abraham Rodriguez, of Lewiston, Maine. That name may have been given to him as part of his witness protection program. A high-ranking police source confirmed the victim was actually Alpo.

But Martinez hardly kept a low profile after his release into witness protection. In a 2019 clip on YouTube, he discussed killing his best friend Porter.

Martinez described how he had his “little man” shoot Porter twice as they were all in a van, but when they went to dump his body, they realized he was still alive. “So I grabbed the gun, I grabbed the gun from my little man and I put one in his head, and then I had to pick him up, I had to pick him up and dump him.”

On Sunday Porter’s family said they finally felt a measure of relief.

“Now my uncle can finally rest in peace, 32 years later,” his niece said. “It’s just that the pain my mother felt all these years, I been feeling the pain with her.”

“Every dog has their day and today was him,” she said of Martinez. “I believe in karma, and I’m glad that I was here to witness it.”

Porter’s family was also honoring the memory of Porter’s younger brother Donnell Porter, who at 12 years old was kidnapped, held for $500,000 ransom, and ultimately murdered before his big brother could come up with the cash.

“I heard boom, boom, boom,” Curd said. “I was sleeping, but that woke me up and then I saw a lot of police was there. ... I don’t know what gun, but it was big shots. Then I heard nothing.”

In a 2020 YouTube interview with Unique, another Harlem drug kingpin from the ‘80s and ‘90s, Martinez explained why he turned federal witness.

“I’m not promoting being a rat, being a government witness. I’m promoting you get locked up, you do what you gotta do. ... Because the code of the streets, that’s where you gonna get poked up. I didn’t live by that code. So where is the code at? I need the rule book,” he told Unique.

“For the youth: I’m not promoting being a rat, I’m not promoting being the biggest thug in the streets. I’m promoting do what you gotta do, and be the best at it.”