The 21-year-old singer, who is a good friend of Kim, said she had warned the rapper to treat the socialite well, reported E! online.
"I've told Kanye before that if he doesn't treat her well, I'm going to come after him," Lovato said.
West, who proposed to Kim, 33, with a 15-carat diamond ring in front of her friends and family, surprised her by popping the question on his knees.
A giant screen on the field flashed sign, 'Please marry me.'