Reed, 23, is best known for her work in "The Twilight Saga" movies.
McDonald, 26, competed on Season 10 of "American Idol."
People.com said the couple posed for photos together at Sunday's MTV Movie Awards.
Reed was wearing a bauble on her left ring finger as she told E! about McDonald. "He's the one," she said.
Kris and Bruce Jenner divorcing
'Vacation' re-boot starts shooting in Georgia