US networks called the race for the New York billionaire shortly after polls closed at 8:00 p.m. (0300 GMT Wednesday).
Sadiq Khan invites Donald Trump to London to meet British Muslims
Early results showed Trump receiving 76 percent of the vote, according to Washington's secretary of state's office.
Trump is the party's presumptive nominee, and while he is not officially the flagbearer yet he is on the cusp.
He had amassed 1,189 delegates prior to Washington state, according to a CNN tally, just 48 delegates short of the 1,237 needed for the nomination.
Washington has 44 delegates up for grabs. Trump will therefore cross the threshold and clinch the nomination on June 7, when California and four other states vote on the final day of the Republican primary contest.
The real estate mogul's former rival, Texas Senator Ted Cruz, as well as Ohio Governor John Kasich and retired neurosurgeon Ben Carson, were still on the Washington ballot even though they have already dropped out of the race.
Take that: Pakistani comedians will roast Donald Trump
Trump, a political neophyte whose provocative campaign has turned American politics on its head, will be officially installed as the Republican presidential nominee at the party's national nominating convention in Cleveland, Ohio in July.
He pivoted to the general election weeks ago, relentlessly criticizing his likely Democratic rival Hillary Clinton.
The former secretary of state has returned fire, although she is still engaged in the final stages of her Democratic battle against challenger Bernie Sanders.
The senator from Vermont has an extremely narrow path to the nomination, and the math clearly favors Clinton.
Washington held its Democratic primary in March, and Sanders won it easily. But Clinton, like Trump, is close to becoming her party flagbearer.
Meanwhile in Albuquerque in the southwestern state of New Mexico, an anti-Trump protest outside his rally at a convention center turned chaotic when protesters began overturning barricades and hurling rocks.
UK's Cameron says would meet Trump despite 'dangerous' Muslim comments
"Protestors are now throwing bottles and rocks at our Police Horses," Albuquerque police tweeted.
Television footage showed some protesters waving Mexican flags.
New Mexico has a large Hispanic population and polls show the demographic is mostly opposed to Trump in large part because of his positions on immigration.
Trump has called for building a wall on the southern US border, deporting millions of undocumented immigrants and banning Muslims from entering the United States.
COMMENTS (1)
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ