"This is unusual and upsetting, but we got some news, we got some news during the show that Robin Williams has passed away," O'Brien said, drawing a gasp from those in attendance.
O'Brien made the announcement at the end of Monday's taping, when the news of Williams' apparent suicide was still fresh.
"Obviously we don't know much yet. We know that this has happened. And we're absolutely stunned, stunned to get this news," O'Brien said.
O'Brien's co-host Andy Richter and actor Will Arnett, a guest on Monday's program, were similarly dejected by the news.
"As funny as he was — he's truly one of the all-time greats — he was even better as a person. He was even more fantastic — just the loveliest, sweetest, one of the kindest guys that I've ever worked with," Arnett said.