"With his extensive private sector experience, his knowledge of economic issues, and his gubernatorial track record, Governor Romney is exactly the kind of leader our country needs to solve our fiscal crisis and restore American prosperity," he said in a statement.
Toomey also praised former Pennsylvania Sen. Rick Santorum, who dropped out of the race this afternoon.
"I also congratulate Senator Santorum on a hard fought primary race," he said. "He put up a valiant fight and deserves to be commended for his spirited effort."
Toomey had a complicated history with Santorum. In 2004, Santorum endorsed then-Sen. Arlen Specter in a GOP primary race against Toomey.
In a Feb. 23 tweet, Toomey said that was "ancient history" and praised Romney's "pro-growth tax reform plan," but he had said he would not endorse in the presidential campaign.