With 55 seconds to go before the break, play was stopped as Williams fell to one knee in front of the Tar Heels' bench. Players, assistants, medical personal and others surrounded him before two individuals lifted him up and walked with him back to the locker room.
He waved to the crowd -- Clemson fans cheered -- as he left the court.
North Carolina officials said at the start of the second half that Williams was feeling better, but he did not return to the game -- an 81-79 UNC win.
"I just wanted to let y'all know I'm alive," Williams said after the game. "I'm not going to croak on anybody. It's vertigo."
Williams, 68, has been open about his bouts with vertigo, which causes dizzy spells in those who suffer from the condition. He suffered a similar-looking incident at Boston College in 2016.
With about a minute to go in the first half on Saturday, Clemson coach Brad Brownell yelled, "Hold up!" to an official as he saw Williams fall to one knee in front of his team's bench.
After Williams left the game, Steve Robinson, his top assistant, coached the remainder of the first half and the second.
Roy Williams says vertigo led to his collapse vs. Clemson
Roy Williams explains he is alright and is "not going to croak on anybody" as it was vertigo that made him fall during North Carolina's game vs. Clemson.
UNC watched Clemson cut a seven-point deficit to two before prevailing.
Williams said he felt well enough to return to the sideline.
"It's excruciating pain for a little while," he said. "I started feeling a heck of a lot better, but he [Robinson] was up six or seven and I didn't want to jinx it. If we had lost, I would have gone back out there with him. But I didn't want to jinx him at that time, and yes, I'm a little superstitious. But I'm so proud of our team, and I'm so proud of this guy sitting beside me for 24 years.
"I told the boys about the movie 'Hoosiers' and how Gene Hackman tries to intentionally get thrown out. I'm only going to [do] this once every two or three years, but I don't do it intentionally."