The former Massachusetts governor defeated Ron Paul, the only other GOP hopeful competing in the state. Rick Santorum and Newt Gingrich did not actively participate in the contest.
Party leaders announced the results Saturday. The caucuses began February 4 and continued through the week. Some Maine communities have yet to hold their caucuses, though party leaders say they don't plan to count those votes.
Romney easily won the Maine caucuses in 2008.
Maine has 24 delegates to the party's national convention - 21 of them were at stake in a process that began with the caucuses.