McCrory announced his backing at the same time Tillis picked up the endorsement of the U.S. Chamber of Commerce at a news conference Tuesday.
McCrory called Tillis a natural leader and problem solver.
Early in-person voting for the May 6 primary ends Saturday.
"I'm proud to announce that tomorrow I plan to vote on the ballot for Thom Tillis for the U.S. Senate," McCrory said. "And in November, I plan to vote for Thom Tillis for the U.S. Senate."
McCrory's move comes a day after the final debate before the May 6 Republican primary. Tillis - the current speak of the North Carolina House of Representatives - faces Baptist pastor Mark Harris, Cary obstetrician Greg Brannon, family nurse practitioner Heather Grant, and four other candidates in the contest.
Two new polls show Tillis in the lead and he has been the leading fundraiser in the GOP primary field and favorite of establishment Republicans in Washington.