With about 60 percent of the ballots counted, Romney had about 83 percent of the vote, according to Puerto Rico's electoral commission. Rick Santorum was in second place with just under 8 percent.
With Puerto Rico's Republican Electoral Commissioner Jose Enrique Melendez declaring Romney's victory official on Sunday night, he was poised to sweep all 20 delegates.
At a town hall meeting in Vernon Hills, Illinois, the former Massachusetts governor said his "extraordinary victory" signaled that Republicans can tap into broad-based support in the Hispanic community.
"Those people who don't think Latinos will vote for a Republican need to take a look at Puerto Rico," he said.
"I intend to become our nominee, and I intend to get Latino voters to vote for a Republican and take back the White House," he added.
Romney has a big lead in support from party delegates, whose backing is needed to win the nomination. But he faces a growing challenge from Santorum in Illinois, which holds its primary contest on Tuesday.
The Illinois vote is the next big hurdle in the months-long fight to win the 1,144 delegates needed to seal the Republican nomination.
Santorum, a former U.S. senator from Pennsylvania, had posed an early potential threat to Romney in Puerto Rico, since his Catholicism and social conservatism were seen resonating among some voters in the predominantly Roman Catholic territory.
But Romney's campaign was endorsed by just about every prominent Republican on the Spanish-speaking Caribbean island, and Santorum angered many Puerto Ricans with comments last week that they needed to make English their primary language if they wanted to pursue statehood over their current status as a self-governing U.S. commonwealth.
"You can't impose English on people. My sense is that he (Santorum) was very poorly advised or he would not have said what he said," Ana Lydia Porrata-Doria, 69, who voted for Romney, told Reuters.
Puerto Ricans, who recognize both English and Spanish as their official languages, will vote in November in a statehood referendum.