Romney also visited with Pacquiao in his dressing room before the fight, wishing him well in the bout.
"Hello Manny. I ran for president. I lost," Romney told the fighter, according to Pacquiao publicist Fred Sternburg.
Pacquiao is a congressman in the Philippines, and has said previously he might run one day for the president of his country.
Romney and his wife, Ann, were guests of Nevada State Athletic Commission chairman Bill Brady at the fight at the MGM Grand hotel arena. Brady hosted a fundraiser for Romney during the presidential campaign.
The Romneys arrived during the undercard, drawing little reaction from the crowd.