"Looking around at all of you, you hard working Iowa families, you farm families and teachers and teamsters and cops and cooks, you rock and rollers and holy rollers!" Palin said. "You all make the world go around and now our cause is one."
The 2008 Republican vice presidential candidate said she was "proud to endorse" Trump.
The official endorsement will be followed by a joint appearance on Wednesday morning in Tulsa, Oklahoma.
The backing could prove pivotal for Trump in Iowa, where he's deadlocked with Ted Cruz in a battle to win the February 1 caucuses traditionally dominated by conservative voters. Palin, a tea party favorite since 2008, has spoken warmly of Cruz in the past and her backing of Trump could blunt the Texas senator's momentum in the final weeks before voting.
The news of the endorsement was first reported by The New York Times
The businessman stoked speculation for much of the day, after an invite
sent around late Monday plugged a special guest who would join him in Iowa. At a press conference on Tuesday, Trump declined to confirm rumors of the Palin endorsement.
"I'm a big fan of Sarah Palin," Trump responded when asked about it by reporters.
In September, CNN's Jake Tapper asked Palin how she would feel about possibly serving in a Trump administration.
"I think a lot about the Department of Energy, because energy is my baby, oil and gas and minerals, those things that God has dumped on this part of the earth for mankind's use, instead of relying on unfriendly foreign nations for us to import their resources," she said, adding that her ultimate aim would to get rid of the agency.
Palin's at times rambling 20-minute speech was filled with phrases like "pussy-footin'," "hallelujah" and "you betcha."
With Trump looking on beside her, she hailed the real estate developer as a "compassionate," "refreshing" and "self-made" man who would "make America great again" in the wake of what Palin described as President Barack Obama's failed tenure.
Trump thanked Palin, calling her a "special" and "amazing" person.
"This is a woman that, from Day 1, I said, 'If I ever do this, I have to get her support,' " the real estate mogul said.
Earlier on Tuesday, Cruz brushed off rumors of the Palin endorsement as a potential blow to his momentum in Iowa.
"Regardless of what Sarah decides to do in 2016, I will always remain a big, big fan," he said.
His campaign also claimed that a Palin endorsement of Trump would backfire on her.
"I think it'd be a blow to Sarah Palin, because Sarah Palin has been a champion for the conservative cause, and if she was going to endorse Donald Trump, sadly, she would be endorsing someone who's held progressive views all their life on the sanctity of life, on marriage, on partial-birth abortion," Cruz spokesman Rick Tyler told CNN's John Berman on "New Day."
Palin responded to the interview Tuesday, linking to a blog post by her daughter Bristol in a tweet.
"Is THIS Why People Don't like Cruz?" the 2008 vice presidential candidate tweeted
In her blog post
, Bristol Palin slammed Tyler for suggesting the former Alaska governor would somehow hurt herself by picking Trump over Cruz.
"After hearing what Cruz is now saying about my mom, in a negative knee-jerk reaction, makes me hope my mom does endorse Trump," Bristol wrote. "Cruz's flip-flop, turning against my mom who's done nothing but support and help him when others sure didn't, shows he's a typical politician. How rude to that he's setting up a false narrative about her!"
The Texas senator didn't mention the speculation about an endorsement for Trump in a tweet Tuesday afternoon, instead saying he will "always be a big fan" of Palin regardless of her role in the 2016 campaign.
"I love @SarahPalinUSA Without her support, I wouldn't be in the Senate. Regardless of what she does in 2016, I will always be a big fan," he said.
Cruz has had a long, contentious history with Sen. John McCain, who ran with Palin in 2008. McCain has called Cruz a member of the "wacko bird" senators and has fueled the argument for those who've questioned whether the Texas senator would even be eligible to serve as president because he was born in Canada.
However, Trump has also spent significant time this cycle trashing the Arizona senator, most visibly this summer when he said that the former prisoner of war was "not a war hero."
Asked at the Capitol in Washington what impact Palin's endorsement would have when people start voting in Iowa, McCain said Tuesday, "I have no idea."
Pressed again what kind of voters Palin's support could bring to Trump, McCain replied, "I don't know -- you'll have to ask pollsters."
Aside from similar conservative populist rhetoric which has inspired fierce loyalty among their working-class supporters, Trump and Palin also have something else in common: Michael Glassner. Glassner worked on Palin's failed 2008 vice presidential bid and Trump hired him as his political director last July