He was leaving a motorcycle dealership in south Florida on Monday when an armed suspect shot him.
Police in Broward County said XXXTentacion, whose real name is Jahseh Onfroy, was taken to hospital where he was pronounced dead.
He was often described as one of rap's most controversial artists and was facing domestic violence charges.
The polarising rapper, who first found an audience by uploading songs to the website SoundCloud, had been hailed as a breakthrough talent and tributes have been pouring in from hip-hop luminaries in the wake of his death.
The county sheriff's office said that Onfroy was leaving the dealership shortly before 16:00 local time when two armed suspects approached.

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At least one of them shot him before both fled the scene in Deerfield Beach, which is 43 miles (69km) north of Miami, in a dark-coloured SUV.
Investigators said it appeared to be a "possible robbery". Video posted to social media appeared to show Onfroy slumped in his car.

A witness told celebrity news website TMZ that multiple shots rang out outside the dealership.
The rapper, whose hit songs include SAD! and Moonlight, quickly rose to prominence following the release of his debut album 17 last August.
His follow-up ? debuted at the top of the Billboard 200 chart in March and has been listened to hundreds of millions of time online.

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The album tackled subjects such as depression and was praised by some of rap's most high-profile stars.
But his short career was plagued by allegations of domestic violence. He was facing 15 felony charges at the time of his death, including aggravated battery of a pregnant woman.

From tough upbringing to number one

XXXTentacion had a troubled upbringing and was expelled from middle school for fighting, but he channelled his energy and fury into music.
He quickly became the most popular artist in the genre known as SoundCloud Rap, defined by its languid, hazy beats and wide-ranging influences.
His surging popularity was noted by the music industry and, by October 2017, he had scored a distribution agreement reportedly worth $6 million (£4.5m).
But the teenager's career was already being overshadowed by his legal problems. Fans were apparently unswayed, sending his latest album to number one in the US.
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Tributes quickly poured in on social media, including from music star Kanye West. "I never told you how much you inspired me when you were here," he posted on Twitter.

Footballer Daniel Sturridge, who has played for Liverpool and England, said it was a "sad day for music".
Rapper J Cole hailed the star's "enormous talent and limitless potential". "God bless his family, friends and fans," he posted.
Documentary maker Louis Theroux said: "Notwithstanding personal demons, he was a huge talent & bringing a beautiful new feel to hip hop. So sad."
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