The announcement, made by Ryan in a column published in his hometown newspaper The Gazette, came almost a month after the highest-ranking GOP officeholder said in an interview with CNN that he was not ready to support Trump.
"Through these conversations, I feel confident he would help us turn the ideas in this agenda into laws to help improve people's lives. That's why I'll be voting for him this fall," wrote Ryan.
"It's no secret that he and I have our differences. I won't pretend otherwise. And when I feel the need to, I'll continue to speak my mind," wrote Ryan. "But the reality is, on the issues that make up our agenda, we have more common ground than disagreement."
Ryan had criticised Trump on several occasions during the primary season, calling him out on his divisive language.
Trump first drew widespread criticism last June when he said in the presidential announcement speech that Mexico was sending "rapists" and drug dealers to the US. Since then, he had repeatedly vowed, if elected president, to deport about 11 million undocumented immigrants in the country.
In another outburst of emotional remarks, Trump called for a "total and complete" ban on Muslims entering the US in the wake of the Paris terrorist attacks in November 2015.
Since then, the targets of Trump's insults expanded to include women, African-American protesters, family members of rivals, etc.
According to a list compiled by The New York Times, Trump had insulted on Twitter 210 individuals, places and things since declaring his presidential candidacy last June.
The Times list did not include targets of Trump's insults broadcasted on cable.
In his interview with CNN in May in which he refused to endorse Trump, Ryan took implicit jabs at the presumptive GOP standard-bearer, urging Trump to run a campaign that would allow Americans to "have something that they're proud to support and proud to be a part of".
"We've got a way to go from here to there," said Ryan.
However, despite the urgency to unite the divided GOP, Trump indicated in his recent interview with The New York Times that he had a "mandate" from his supporters to continue his candidacy as a "fiery populist outsider".
"You win the pennant and now you're in the World Series -- you gonna change?" said Trump to The Times. "People like the way I'm doing."