The property billionaire said he was "greatly honoured" by the endorsement
Sarah Palin, the former Alaska governor and vice presidential candidate, has backed Donald Trump in his run for the White House.
"I'm proud to endorse Donald J Trump for president," Mrs Palin said in a statement on his campaign website.
The property billionaire said he was "greatly honoured" by the endorsement from his fellow reality TV star.
"She is a friend, and a high-quality person whom I have great respect for. I am proud to have her support," he added.
She announced her endorsement in person on stage alongside Mr Trump at a rally in Ames, Iowa.
Mrs Palin told the audience: "He's the master of the art of the deal. He's beholden to no one except we the people. No more pussyfooting around".
"He's from the private sector, not a politician. Can I get a hallelujah?".
Mrs Palin's seal of approval comes at a pivotal moment as the countdown begins to the first-in-the-nation presidential primary, on February 1st, in the Hawkeye state.
Her endorsement is a blow to one of Mr Trump's closest rivals, Texas Senator Ted Cruz, who is also wooing disaffected, anti-establishment voters.
He sought to put a brave face on the high-profile endorsement of his arch-rival for the Republican presidential nomination.
He praised Mrs Palin saying "without her support, I wouldn't be in the Senate", in reference to his election in 2012.
"Regardless of what she does in 2016, I will always be a big fan," he tweeted.
But his spokesman Rick Tyler told CNN's New Day programme that he would be "deeply disappointed" if she backed Mr Trump.
Though her conservative star power has waned in recent years, Mrs Palin still enjoys a popularity among tea party voters that could boost Mr Trump in a dead-heat against Mr Cruz in Iowa.
Mr Cruz's campaign was also rocked on Tuesday as Iowa's governor warned voters it would be a "big mistake" to pick him.
Terry Branstad, a Republican, cited Mr Cruz's opposition to government support of biofuel in the corn-producing state.