The newspaper's editorial board said it decided Romney was better equipped than President Barack Obama to deal with the economy and a sometimes-stubborn Congress.
"Which candidate could forge the compromises in Congress to achieve these goals?" the editorial said. "When the question is framed in those terms, Mitt Romney emerges the stronger candidate."
The Register endorsed Obama in 2008 and said it still considered the president "superbly qualified." But the editors said they leaned toward Romney because of his successes in the business world, the Salt Lake City Olympics and as the conservative governor of the liberal state of Massachusetts.
"The Register's editorial board, as it should, had a vigorous debate over this endorsement," the newspaper said. "Our discussion repeatedly circled back to the nation's single most important challenge: pulling the economy out of the doldrums."
The last time the Register endorsed a Republican was 1972, when it went for Richard Nixon.