In February, Eastwood told Fox News that he wasn't supporting any politician at that time. Some saw the "halftime in America" ad he made for the Super Bowl as a nod toward President Barack Obama. Eastwood responded then by saying he was not "politically affiliated" with the president.
"Now more than ever do we need Gov. Romney. I'm going to be voting for him," Eastwood told Romney supporters Friday night.
"He just made my day," Romney said. "What a guy."
Standing at Romney's side, Eastwood said he was filming "Mystic River" in Massachusetts almost a decade ago when he first saw political advertisements featuring Romney, who was running for governor at the time.
"I said, God, this guy, he's too handsome to be governor, but he does look like he could be president," Eastwood joked. "As the years have gone by I began to think even more so about that."
Eastwood, 82, said he hoped Romney would restore "a decent tax system that we need badly ... so that there's a fairness and people are not pitted against one another as who's paying taxes and who isn't."
The actor and director instantly became Romney highest-profile celebrity supporter. Romney has also earned the endorsement of Oscar-winning actor Jon Voight and rock star Kid Rock.
A campaign spokesman could not immediately say whether Eastwood would play an active role in the campaign.
About 325 people paid as much as $25,000 apiece to attend the event. Eastwood is a part-time resident of Sun Valley.