Romney is pitching himself to be an alternative who can fix the country's ailing economy. He says the time is ripe for his candidacy since the economy and business are his so-called "power alley."
The former Massachusetts governor made his announcement at a farm in New Hampshire.
Romney, the closest thing to a frontrunner in a Republican field that lacks one. He lost to John McCain in the 2008 presidential primaries.
During his speech, Romney said his first priority will be to make America the world's top job creator.
The Associated Press contributed to this story.