He said in a social media message, "after much prayer and discussion with family and friends I've decided to retire at the end of this term."
Hatch, the Senate's longest serving Republican, has wrestled with the decision for months, emboldened by the entreaties of President Donald Trump to seek an eighth term.
Trump and Romney have rarely seen eye-to-eye, with Romney dealing harsh criticism in the lead up to 2016's presidential election:
"Here's what I know. Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University," Romney said in a March 2016 speech. "He's playing the American public for suckers: He gets a free ride to the White House and all we get is a lousy hat."
The former governor of Massachusetts wished Hatch well in his retirement but made no mention of running to fill his seat.
I join the people of Utah in thanking my friend, Senator Orrin Hatch for his more than forty years of service to our great state and nation," Romney said in a Facebook post. "Ann and I wish Senator Orrin Hatch and his loving wife Elaine all the best in their future endeavors."