Rubio's endorsement came in an interview on Fox News Wednesday, CNN reported.
"It's evidently and increasingly clear that Mitt Romney is going to become the Republican nominee," Rubio said. "We have to come together behind who I think has earned the nomination, and that's Mitt Romney."
Rubio, who has been mentioned as a possible vice presidential candidate, told MSNBC Wednesday he won't be the nominee for the post.
"I'm not going to be the vice presidential nominee but I'm always flattered when people bring it up," he said. "I think they mean it as a compliment."
Former Florida Gov. Jeb Bush, who has endorsed Romney, is among those who said he hopes Rubio is picked as the vice presidential nominee.
On Fox, Rubio said a floor fight over the nomination at the Republican National Convention would hurt the GOP.
"There is no way that a floor fight at the convention in Tampa in August is a recipe for a win in November," Rubio said. "On the contrary, I think it's a recipe for disaster."