This would be Romney's second run for the White House after losing the Republican nomination to Sen. John McCain in 2008.
In his announcement, Romney focused on the economy and what he described as President Barack Obama's failed policies.
"It is time that we put America back on a course of greatness, with a growing economy, good jobs and fiscal discipline in Washington," the former Massachusetts governor said in a video posted on his website, on Facebook and on Twitter.
He criticized Obama's stewardship of the economy and offered a preview of his expected campaign theme: Romney is a proven business executive while Obama remains unqualified to lead.
"(Obama) and virtually all the people around him have never worked in the real economy. They just don't know how jobs are created in the private sector," Romney said.
Romney, a businessman who ran Salt Lake City's Winter Olympics, has lined up donors, staff and advisers for his second presidential bid.
The Associated Press contributed to this story.