The ad, entitled "At Stake", is a 30-second spot from the super PAC American Crossroads, which supports Republican candidates around the country and is run by Karl Rove.
"In the last few years, America's been knocked down," Eastwood's distinctive raspy voice says over images of an abandoned factory and unemployed workers.
"When someone doesn't get the job done, you've gotta hold them accountable," Eastwood continues. "We need someone who can turn it around fast and that man is Mitt Romney."
He then urges people to act fast because "the future of our country is at stake."
Eastwood's last foray into the political spotlight was during the Republican National Convention in early August, with his unscripted comedy routine with an empty chair. While the convention crowd ate it up, some questioned why the Romney campaign used more than 10 minutes of their primetime coverage on something not focused on the candidate.
According to the New York Times, the ad will start running Wednesday in seven swing states: Colorado, Florida, Iowa, Nevada, New Hampshire, Ohio and Virginia.