Grimmie was a YouTube sensation and a former contestant on The Voice. The gunman who shot Grimmie travelled to her Florida gig specifically to attack her, said police.
Kevin Loibl was carrying two handguns, two loaded magazines, and a hunting knife, when he entered The Plaza Live concert venue.
Grimmie was signing autographs when Loibl opened fire. Her brother, Marcus, immediately tackled the gunman, who then shot and killed himself during the struggle.
She was taken to hospital after the attack and died early on Saturday.
Orlando Police chief John Mina said that Loibl, of St Petersburg, Florida, did not appear to know Grimmie personally.
He said: “She was doing a meet-and-greet, just signing autographs and selling merchandise. This white male approached her and opened fire, striking her.
“We believe he came here to commit this crime.”
Grimmie, of New Jersey, finished third during season six of NBC’s The Voice in 2014, competing on the team of Maroon 5 star Adam Levine.
Earlier, police said she had performed with the band Before You Exit at the gig, which ended around 10pm, and was shot as she signed autographs for fans at a merchandise table.
Marcus Grimmie’s actions in bravely tackling Loibl stopped the gunman from hurting others, police said. Around 120 people were in The Plaza Live at the time.
“Very heroic actions by Marcus Grimmie to jump in and it definitely could have prevented further loss of life,” Mr Mina said.
Detectives were searching Loibl’s mobile phone and social media accounts looking for clues as to a motive, the police chief said.
At Loibl’s home in St Petersburg, someone had left a note on the front door, expressing “deepest sorrow” for the loss “to the family, friends & fans of the very talented, loving Christina Grimmie”. It said there would be no other comment.
Loibl had made travel arrangements to come to Orlando alone, as well as to go back home, but he did not have a car, Mr Mina said.
There were unarmed security guards at The Plaza Live and they checked bags for contraband, but there were no metal detectors or pat-downs of people, he said.
The Voice paid tribute to Grimmie on its Twitter page: “There are no words. We lost a beautiful soul with an amazing voice.”