The republican candidate made it official during a campaign event on a farm in New Hampshire.
Romney went right after President Obama, saying the U.S. is in peril and that Obama has failed to solve the nation's serious economic problems.
He also accused Obama of crafting a foreign policy based on apologizing for the United States.
The 64-year-old Romney was an unsuccessful presidential candidate in 2008.
Romney defended his signature achievement as governor of Massachusetts.
The controversial healthcare reform bill that President Obama pushed through Congress was modeled after legislation that Romney signed as governor of Massachusetts.
Romney called it an imperfect solution to a specific state problem.
He also called for a complete repeal of the national healthcare reform legislation.
Republican rivals are expected to pound Romney on healthcare as the GOP primary season heats up next year.