The 22-year-old "All About That Bass" singer nabbed the coveted award and took to the stage crying as this is her first Grammy nomination and award since releasing her first album early last year.
Trainor cried throughout her acceptance speech.
"I have to thank LA Reid for looking at me as an artist instead of just a songwriter," she said while accepting the award from presenter Sam Smith, who won the award last year.
"And my mom and dad for always believing in me," Trainor added. Her father also got emotional.
The singer beat out Sam Hunt, James Bay, Tori Kelly and Courtney Barnett to take home the award.
During the show, Trainor joined Demi Lovato, John Legend, Luke Bryan and Tyrese on stage for a tribute to Lionel Richie.