The victim was identified as Kevin Mares, a 25-year-old who lived in New York, according to a police statement.
The shooting happened in the predawn hours of Sunday at a nightspot called "Refuge for Mistreated Men" in La Perla, a coastal community that has struggled to shed its dark reputation.
Homicide detective Sergeant Arnaldo Ruiz said in a phone interview that the shooting took place when several people near Mares began arguing and one pulled out a gun and shot at least three people, including Mares. A brother and a sister in their mid-40s who live in La Perla were injured and remain hospitalised.
Ruiz said Mares was an innocent bystander. He was with three other friends who told police they were in Puerto Rico for one of Bad Bunny's 30 concerts, which have attracted tens of thousands of visitors to the US territory.
Mares was shot on the left side of his abdomen and was taken to Puerto Rico's largest public hospital, where he died, authorities said.
Ruiz said police don't yet know what the people were arguing about and don't have a description of the shooter. "We have very little information," he said.
Ruiz added that Mares' three friends also were from New York. He didn't know their hometowns.
La Perla is located on the outskirts of a historic district popular with tourists known as Old San Juan. A couple hundred people live in the shantytown, which once served as Puerto Rico's biggest distribution point for heroin and was known for its violence.
Police used to avoid the community, which used to have a sign proclaiming, "Not open to visitors. Do not enter."
But violence eased when hundreds of federal agents raided the slum in 2011 and arrested dozens of people, including a well-known community leader who was later convicted.
The neighbourhood became even safer and more welcoming after Puerto Rican singers Luis Fonsi and Daddy Yankee featured it in their hit, Despacito.
But isolated, violent incidents persist.
In February 2023, three tourists were stabbed after police said a person told them to stop filming inside the community.
Then in April 2024, a 24-year-old tourist from Delaware was killed and his body set on fire after police said he and a friend were attacked following a drug purchase. Police said the victims were trying to take pictures of La Perla after being warned not to do so.
The island of 3.2 million people has reported 277 killings so far this year, compared with 325 killings in the same period last year.
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