With 84 percent of precincts reporting in Hawaii, Romney won 45 percent of the state's votes. Former Sen. Rick Santorum finished second with 25 percent, followed by Rep. Ron Paul at 19 percent and former House Speaker Newt Gingrich at 11 percent.
The island sweep provided a needed lift on a night that saw Romney fail to win two key Southern primaries in Mississippi and Alabama, states where he had hoped to deliver a knockout punch to his rivals.
Romney instead finished third in both states, which were won by Santorum.
In Hawaii, both Santorum and Paul invested resources in the state. with Paul buying television ad time there and Santorum spending on robocalls.
Santorum's daughter, Elizabeth, also campaigned for her father on the day of the caucuses, meeting with college Republicans and also visiting the state capitol, both in Honolulu. Likewise, Paul's son Ronnie also campaigned for his father in the state, visiting a school in Honolulu.
Romney was expected to win in the state, which has a large Mormon population, and had sent his son Matt to campaign on his behalf.
Romney has swept nominating contests in U.S. territories. Last Saturday, he won caucuses in Guam and the Northern Mariana Islands, as well as most delegates from the U.S. Virgin Islands. Romney is also expected to win in Puerto Rico's winner-take-all primary, the next contest in the GOP calendar.
Hawaii, a solidly blue state, backed native son Barack Obama in the 2008 presidential election.
—Daniel Strauss contributed.