Romney, who with his wife Ann was a ringside guest of Nevada State Athletic Commission chairman Bill Brady, visited Pacquiao in his dressing room during one of the fights on the undercard at the MGM Grand Garden Arena.
"I wish you good luck tonight," a smiling Romney said in front of television cameras to Pacquiao, who has won world titles in an unprecedented eight weight divisions. "Have a great night. Good to see you."
According to Pacquiao's publicist Fred Sternburg, Romney began the short conversation by saying: "Hello Manny. I ran for president. I lost."
Pacquiao is no stranger to politics, having claimed a seat in his country's national congress in May 2010 when elected to the House of Representatives in the 15th Congress of the Philippines.
On Saturday, however, the 33-year-old Filipino southpaw will be aiming for bold vindication in the ring when he fights Mexican Marquez for a fourth and final time.
The two boxers fought to a draw in May 2004 before Marquez lost his WBC super-featherweight title to Pacquiao in a controversial one-point split decision in March 2008.
When they last met, in November last year, Pacquiao narrowly retained his WBO welterweight title with a controversial majority decision that was greeted by loud boo-ing from disgruntled Marquez fans.
"He (Marquez) always claims he won the fights," said Pacquiao, who has a career record of 54-4-2 with 38 knockouts. "So he needs to prove something.
"I am giving him a chance to prove he can win the fight because he thought he has won all three and he keeps talking about it.
"So it is very important to me, to win this fight, especially since Marquez really wanted this fight," said the Filipino who lost his most recent fight on a hotly disputed split decision to American Timothy Bradley in Las Vegas in June.
(Reporting by Mark Lamport-Stokes; Editing by Ian Ransom)