Levine said that those who have been "nasty" about Aguilera's weight gain, should "grow up" and stop bullying her, reported People magazine.
"It's not cool. I've never liked it. I think it's horrible and I'll totally defend Christina when people say things like that because it's no-one's business," Levine said.
"The one thing about the culture right now, celebrity culture particularly, is people feel like they can just say nasty things about other people whether it's Christina or whether it's me. She gets a lot and it pisses me off. Of course I have her back and of course I defend her," he added.
The two singers are both coaches on singing reality show 'The Voice' and the pair have often bickered and had disagreements on the NBC reality show in the past.
However, Levine said that the fights were "fictional".
"We never hated each other. We never had these secret battles that everyone thought we were having," he said.
Aguilera is taking a hiatus from 'The Voice' this season along with fellow judge Cee-Lo Green, to be replaced by Usher and Shakira.