The Republican presidential nominee had previously said he would not back Ryan's bid to become re-elected as House Speaker, but reversed his position at a rally in Green Bay, Wisconsin, saying: "We may disagree on a couple of things but mostly we agree."
Reading from a sheet of paper, Trump said: "In our shared mission to make America great again, I support and endorse our speaker of the House, Paul Ryan."
Ryan has called out Trump's demand for a ban on Muslims entering America, tweeting on July 31: "A religious test for entering our country is not reflective of America's fundamental values. I reject it."
The House speaker also recently described the would-be President's attack on a federal judge as "the textbook definition of a racist comment".
But as the highest-ranking Republican in Washington, Ryan is hugely influential - something Trump has clearly recognised.
It's been a long journey for Trump, however, who told the Washington Post on Tuesday: "I like Paul, but these are horrible times for our country.
"We need very, very strong leadership. And I'm just not quite there yet. I'm not quite there yet."
Trump's language was reminiscent of the words Ryan used when he refused to back Trump in May, saying: "I'm just not ready to do that at this point. I'm not there right now."