The decision, announced by the House of Representatives speaker in a column for a newspaper in his home state of Wisconsin, ended a long period of soul-searching over whether to back Trump out of concern about his bellicose rhetoric and positions on issues including trade and immigration.
Ryan's support for Trump surfaced in the middle of a speech by Democratic front-runner Hillary Clinton in which she launched a far-reaching attack on Trump's foreign policy credentials.
The speaker's backing of Trump could give cover to more reluctant Republicans to get behind the billionaire businessman as their best chance to win the White House.
It should also help Trump make the case that he can bring the party together as he girds for a Republican nominating convention in July that many party leaders plan to skip.
Ryan was the 2012 vice presidential running mate of Mitt Romney, who has been sharply critical of Trump.
"It’s no secret that he and I have our differences. I won’t pretend otherwise," Ryan wrote of Trump. "And when I feel the need to, I’ll continue to speak my mind. But the reality is, on the issues that make up our agenda, we have more common ground than disagreement."
Ryan did not specifically use the word "endorse" in his column, but his spokesman, Brendan Buck, made clear that Ryan's move should be seen as an endorsement.
Ryan said he and Trump had spoken many times in recent weeks about how, "by focusing on issues that unite Republicans, we can work together to heal the fissures developed through the primary."
"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," Ryan said.