Mr Trump shocked party members on Tuesday when he refused to endorse Mr Ryan and Senator John McCain in their campaigns for re-election in November.
Mr Ryan, the party's highest-ranking elected official, had himself long delayed endorsing Mr Trump.
"I support and endorse our Speaker... Paul Ryan," Mr Trump said on Friday.
"We need unity - we have to win this election," Mr Trump said during a campaign stop in Green Bay, Wisconsin - Mr Ryan's home state.
While they might "disagree on a couple of things", they agreed on a lot as well, he added.