The `Iron Man 2` star - who married the movie`s producer Susan Levin in 2005 - credits his spouse for helping him turn his back on drug addiction and can-t imagine ever living without her.
He said: "I don`t joke when I say she is my better half. She truly is.
"I cannot believe what a fantastic life I have today. But here`s the thing, she didn`t change me at all. She just gave me an ultimatum at a certain point.
"Ultimately, we both changed a lot so that now I can`t ever imagine a life before her. I`ve certainly come to believe that Mrs. Downey and I together are definitely better than me alone or me and anybody else."
The actor also believes his role as Tony Stark in 2008-s -Iron Man- finally helped him turn around his image, and hails his transformation as a miracle.
He added to The Independent newspaper: "It-s pretty miraculous. Although I-m not sure the public-s opinion of me changed, it was more of a movie industry thing.
"Nonetheless, it-s mind-blowing to me because I always felt like I could do something like this, even when nobody else agreed. It-s wild."