TMZ is reporting that Onfroy has been dubbed "a Level 1 trauma patient" and is comatose on his way to the hospital.
XXXTentacion, 20, appeared lifeless in his auto in video taken right after the shooting.
"Dispatchers received a call of a shooting at 3:57 p.m.", the tweet continued.
United States rapper XXXTentacion, who shot to fame with consecutive hit albums, has been killed aged 20.
#BreakingNews The adult male that was taken to the hospital has been pronounced dead.
XXXTentacion, also known as Jahseh Dwayne Onfroy poses for his mugshot after being charged with seven new felonies stemming from a 2016 domestic violence case on December 15, 2017 in Miami.
Witnesses told TMZ.com that the genre-blending rapper was leaving the dealership parking lot in a black BMW when shots were fired from another auto.
In his few years in the spotlight, the 20-year-old Florida native saw national success as a rapper as well as several high-profile run-ins with the law, including allegations he was abusive.
In the last 18 months, Onfroy quickly became one of popular music's most controversial and, in some circles, reviled figures.
The rapper has had a troubled past and was one of the artists - along with R.Kelly - to have his music taken off the streaming service Spotify before Kendrick Lamar intervened.