Trump garnered 76 percent of the vote. Texas Sen. Ted Cruz and Ohio Gov. John Kasich each earned 10 percent of the vote, while retired neurosurgeon Ben Carson earned 4 percent of the vote. In all, approximately 115,000 votes out of 482,000 were cast for former Trump rivals.
Trump's convincing victory came days after Washington state's GOP convention awarded 40 of its 41 elected delegate slots to Cruz supporters. However, under party rules, each delegate is bound to the primary results for the first round of voting at the national convention.
Republicans in Washington were to allocate all 44 delegates to this summer's national convention in Cleveland based on the primary results. Trump had secured at least 27 delegates as of late Tuesday, leaving him 41 short of the number needed to clinch the nomination, Fox News reported.
Trump is expected to easily secure the nomination on June 7, when GOP contests are held in California, Montana, New Jersey, New Mexico and South Dakota, with a total of 303 delegates at stake.
Washington has both a presidential primary and a caucus system. But Democrats will ignore the results of Tuesday's primary, which frontrunner Hillary Clinton was projected to win, having chosen to use the party caucus system to allocate their national convention delegates.
Vermont Sen. Bernie Sanders handily won the Washington Democratic caucuses in March, receiving 74 delegates to the national convention in Philadelphia to Clinton's 27.
Clinton is just 78 delegates short of clinching the Democratic nomination for president. She is on track to do so in early June, even if she loses all the remaining contests.
When superdelegates are included, Clinton has 2,305 delegates and Sanders has 1,539. It takes 2,383 delegates to win the Democratic nomination.