22-year-old Christina Grimmie came third on the American version of the show in 2014
Christina Grimmie, who came third on the show in 2014, was targeted while signing autographs after performing at the Plaza Live venue in Orlando.
The unidentified shooter killed himself after being tackled by the singer's brother, whose actions police say prevented others being hurt.
With deep regret, we have confirmed Christina Grimmie, @TheRealGrimmie has died from her injuries.— Orlando Police (@OrlandoPolice) June 11, 2016
The 22-year-old's publicist, Heather Walsh, later added: "We can confirm that Christina has passed and went home to be with the Lord."
Police said a man with two guns walked up to the 22-year-old near a merchandise table and opened fire.
Grimmie was hit at least once.
The YouTube star had just opened for the band Before You Exit when the shooting occurred about 10.30pm local time.
Witnesses saw people trying to give her CPR at the scene and she was initially described as being "critical" in hospital.
Concert goer Juana Villegas told the Orlando Sentinel: "I don't think anyone expected something like this to happen. It was horrifying.
"Then a girl comes out hysterically crying and says someone shot themselves next to her."
Hot dog stand worker Josh Call heard four or five gunshots.
"It was quick like pow, pow, pow, pow," he said, adding that a security guard rushed to clear others from the area.
On Twitter, #RIPChristina became the top trending hashtag as fans took to social media to express grief.
The Voice paid tribute on its official Twitter page: "There are no words. We lost a beautiful soul with an amazing voice."
There are no words. We lost a beautiful soul with an amazing voice. Our hearts go out to the friends, fans and family of @TheRealGrimmie.— The Voice (@NBCTheVoice) June 11, 2016