CNN reported that with 52 percent of the U.S. territory's precincts counted, Romney had more than 61,000 votes, or 83 percent. Rick Santorum was second with 8 percent, or about 5,600 votes. Newt Gingrich was third with 2 percent and Ron Paul was fourth with 1 percent.
The results were obtained from local party and election officials, CNN said.
Romney told supporters at a rally in Vernon Hills, Ill., Sunday evening the results in Puerto Rico make clear which of the four GOP candidates "most represent their feelings." He said the Republican platform for the economy, including smaller government, lower taxes and being pro-business can appeal to Latinos and the rest of the country.
"Those people who don't think that Latinos will vote for a Republican need to take a look in Puerto Rico," Romney said. "Hispanic voters are going to vote for Republicans if we stand for something -- conservative principles that bring growth and good jobs and rising home values. That's how we're going to win, and we're going to get Latino voters to help us out."
Puerto Rico will send 20 delegates to the Republican National Convention. Romney will get all 20 as long as he finishes with 50 percent or more of the total vote.
CNN said with his win in Puerto Rico it estimates Romney has secured 518 delegates so far this primary season to Santorum's 239. Gingrich is 100 delegates behind Santorum and Paul has 69 delegates. A candidate needs 1,144 delegates to secure the party's nomination.
Observers said the candidates' stance on Puerto Rican statehood was a major factor in the primary, where a larger-than-normal turnout of about 400,000 people was expected, observers say.
Citizens of the commonwealth cannot vote in the November general election.
Romney, a former Massachusetts governor, had certain advantages heading in the primary, The New York Times reported, including his unambiguous support of statehood and an endorsement from the governor, Luis G. Fortuno.
While the territory is split on the issue of U.S. statehood, the New Progressive Party, which aligns with the Republican Party, supports becoming the United States' 51st state.
All three of the major candidates support Puerto Rico's statehood, but Santorum, a former U.S. senator from Pennsylvania, came under fire for comments he made suggesting English be the main language on the island, the Los Angeles Times reported Sunday.
Santorum said his statement was a misunderstanding and that he merely wanted Puerto Rican citizens to speak both English and Spanish. But one delegate broke ties with him over the incident.
"It really hurt him," Puerto Rico's Republican National Committeewoman Zoraida Fonalledas said Friday. "If [people] were going to vote for him, they changed their minds."