"In our shared mission to make America great again, I support and endorse our Speaker of the House, Paul Ryan," Mr Trump said in his annoucnement at a rally in Green Bay, Wisconsin.
"We may disagree on a couple of things," he continued, trailing off, "but mostly we agree and we're going to get it done and we're going to do a lot of wonderful things."
Mr Trump announced the endorsement while calling for unity in the Republican party, in an effort to mend the rift that his success throughout the election season has caused. He stressed the importantce of growing the Republican majority in the House and Senate, in a roundabout acknowledgement that he will need their help to push through his policies.
"I need a Republican Senate and a House to accomplish all the changes that we have to make," he said. "I understand and embrace the wisdom of Ronald Reagan ... that my 80 per cent friend is not my 20 per cent enemy – Ronald Reagan – stated by Ronald Reagan. Pretty good."
Mr Ryan, the highest-ranking Republican official, faces a Tuesday primary in his home state of Wisconsin against Paul Nehlen.
In addition to endorsing Mr Ryan, Mr Trump also walked back his non-endorsement of Arizona Sen John McCain.
"While I'm at it, I hold in the highest esteem Senator John McCain, for his service to our country in uniform and in public office," he said. "And I fully support and endorse his reelection. Very important. We'll work together."
Previously, Mr Trump drew criticism when he publicly mocked Mr McCain for his time as a prisoner during the Vietnam War, saying: "I like people who weren't captured."
In a Tuesday interview with the Washington Post, Mr Trump declined to endorse both Mr Ryan and Mr McCain.
“I like Paul, but these are horrible times for our country. We need very strong leadership. We need very, very strong leadership,” he said. “And I’m just not quite there yet. I’m not quite there yet.”
Mr Trump criticised Mr McCain for not doing "a much better job for the vets".
Indiana Gov Mike Pence, Mr Trump's running mate, announced his endorsement of Mr Ryan a day after the Post interview went to print.
“I strongly support Paul Ryan, strongly endorse his reelection,” he told Fox News. “He is a longtime friend. He’s a strong conservative leader. I believe we need Paul Ryan in leadership in the Congress of the United States.”
Mr Trump’s refusal to provide an endorsement seemed to indicate a sense of retaliation against Mr Ryan, who hesitated to back the New York businessman prior to his receiving the Republican nomination.
“I’m just not ready to do that at this point,” Mr Ryan told CNN in May. “I’m not there right now.”
Mr Ryan was reportedly not privy to the fact that Mr Trump had planned to announce his endorsement on Friday, and said they have not spoken to each other since the Republican Convention.
On top of that, he told a Milwaukee talk radio host that while he had maintained his endorsement of the Republican nominee thus far, it is not "irreversible".
"With any endorsement of anybody, there’s never a blank check, and you know that and that’s how I’ve always felt," Mr Ryan said on WTMJ radio. “I see no purpose in doing this tit-for-tat, petty back-and-forth with Donald Trump, because it serves no good purpose in my mind.”
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