The Florida senator said he apologised privately to Trump in person backstage before a Republican debate for the crude remarks he made about Trump's appearance.
"I actually told Donald, one of the debates, I forget which one, I apologised to him for that. I said, you know, I'm sorry I said that. It's not who I am and I shouldn't have done it," Rubio told CNN.
During his final weeks as a Republican candidate, struggling to catch up to 69-year-old Trump in the polls in his home state of Florida, Rubio made a joke about Trump's "small hands" that implied the size of other body parts as well.
Meditating on everything from Trump's rise to his fractious relationship with Jeb Bush, Rubio in an interview aired yesterday revisited nearly every turn of his presidential run.
The former presidential candidate, who has grudgingly said he will support Trump in November, also admitted a series of mistakes that he said eventually bedevilled his campaign.
Rubio said he would be willing to speak on Trump's behalf at the convention and also signalled some respect for the man he had sharply criticised.
Rubio recommended that Trump not drastically alter his approach in the general election.
In another sign of the two leaders warming up to each other, Trump last week tweeted that Rubio should seek re-election as Florida Senator.
"Poll data shows that @marcorubio does by far the best in holding onto his Senate seat in Florida. Important to keep the Majority. Run Marco!" Trump tweeted.
