In his tweet, which linked to an article on the website for The Janesville Gazette (his hometown paper), Ryan argued that Trump would "help turn the House GOP's agenda into laws."
A spokesman for Ryan clarified on Twitter that the speaker's team is "not playing word games," and others should "feel free to call it an endorsement."
In his piece for the Gazette, Ryan wrote that a White House led by Democratic front-runner Hillary Clinton "would mean four more years of liberal cronyism and a government more out for itself than the people it serves." Trump, Ryan wrote, would be a more likely ally.
"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," Ryan wrote. "But the reality is, on the issues that make up our agenda, we have more common ground than disagreement."
Ryan had equivocated on a Trump endorsement for weeks, saying in May that he was "not ready" to fully endorse the New York businessman.
Soon after Ryan's announcement, his move was applauded by former House Speaker Newt Gingrich.