That came as a surprise to many, as the former Massachusetts governor has been a frequent and harsh critic of Trump, and the now-president has called Romney "a total joke."
Romney, who announced he would run for Senate in Utah on Friday, expressed his gratitude for the endorsement.
That acceptance itself was incongruous with what Romney had said in 2016 about Trump's endorsement from the prior presidential election.
The 2012 Republican presidential nominee is fighting criticism that he is an outsider for the Senate seat — he mentioned Utah early and often in a video message announcing his candidacy.
"I have decided to run for United States Senate because I believe I can help bring Utah's values and Utah's lessons to Washington," he said. "Utah is a better model for Washington than Washington is for Utah."
Romney is aiming to replace 83-year-old Sen. Orrin Hatch in November's election. The longtime senator announced his retirement in January even as Trump pushed him to run again.
Romney heavily criticized then-candidate Trump in a 2016 speech, calling him a "phony" and a "fraud." He warned that Trump would cause economic instability and endanger Americans abroad.
Later, Romney unsuccessfully interviewed to be Trump's secretary of State. Since then, he has publicly rebuked Trump when he supported Roy Moore, the Alabama Senate candidate accused of sexually abusing teenagers, and when the president reportedly questioned why the U.S. needed immigrants from "s---hole" African countries.
For his part, Trump has also been critical of Romney, saying in the past that he "choked like a dog" during the 2012 run for the White House, and that he is "a mixed up man who doesn't have a clue."
Some of Trump's criticism of Romney:
If he wins the seat and criticizes Trump while in office, Romney would mark a stark shift from Hatch. Hatch has heaped praise on the president in recent months, calling him a "heck of a leader" after the GOP passed its tax plan in December. Trump reportedly begged the 83-year old Hatch to run for re-election one more time.
This is a breaking news story. Check back for updates.
—CNBC's Jacob Pramuk contributed to this report.