Eastwood attended a Romney fundraiser in Sun Valley, Idaho, said to have raised over $2m (£1.29m).
The Dirty Harry star said he was endorsing the Republican because "the country needs a boost somewhere".
In February Eastwood starred in a Chrysler Superbowl advert, Halftime in America, sparking debate over whether he backed President Barack Obama.
At the time, Eastwood had said he was not endorsing either candidate and, speaking to Fox News, said he was "certainly not politically affiliated with Mr Obama".
"It was meant to be a message about just about job growth and the spirit of America," Eastwood said in February of the Super Bowl advert.
"I think all politicians will agree with it. I thought the spirit was OK. I am not supporting any candidate at this time."
'Handsome enough for president'?
On Friday Mr Romney said of Eastwood's endorsement: "He just made my day. What a guy.
After an introduction from Mr Romney, Eastwood spoke to about 325 guests gathered for the Idaho fundraiser.
Eastwood said he first saw Mr Romney when he was running for governor of Massachusetts, and Eastwood was directing the film Mystic River in Boston.
Eastwood said he thought: "God, this guy, he's too handsome to be governor. But it does look like he could be president."
Backing Mr Romney, Eastwood added that Mr Romney would "restore a decent tax system... so there's a fairness and people are not pitted against each other," according to reports from Idaho.
Mr Romney is challenging Mr Obama for the White House in November's presidential elections.
Also on Friday figures showed that the US added 163,000 jobs in the month of July, but national unemployment rose to 8.3% from 8.2%.
Correspondents say the health of the US economy is likely to become a decisive factor in the outcome of the election.
OP: Would anyone seriously go 'I don't know who to vote for...oh Clint likes him so I guess its Romney for me'?
Its interesting for me too because although the Labour Party in the UK have celebs that endorse them and appear in their adverts, nobody really comes out and declares their undying love for the Conservatives. They do it quietly of course but not in any big way.