REUTERS/Carlo Allegri
Sarah Palin.
"I'm proud to endorse Donald J. Trump for president," the former Republican vice-presidential nominee said.
Trump had repeatedly teased an endorsement event on Tuesday evening in Ames, Iowa.
The Republican front-runner said earlier this week that he'd have a "major announcement" and a "very special guest" for his rally there.
Political observers had widely expected that Trump would be endorsed by Palin, who has spoken fondly about Trump's campaign. Much of the Republican establishment shunned Trump after his various caustic statements and populist policy announcements, leaving Palin as arguably Trump's highest-profile backer to date.
Trump said in a statement that he was "greatly honored" to have her support.
"She is a friend, and a high-quality person whom I have great respect for," Trump said. "I am proud to have her support."
US Sen. John McCain (R-Arizona), Palin's former running mate, has been among Trump's more prominent critics. Palin's star has dimmed somewhat since her 2008 campaign and sudden exit from the Alaska governorship, but she remains popular with a large number of conservatives.