"We, like you, were a bit shaken up last night when we learned that genius comedian and actor Robin Williams passed away," said Fallon with a tremble in his voice. "He was one of a kind, one in a million. He's like unbelievable."
Fallon paid his respects to Williams by doing an impersonation of the late comic's stand-up routine.
"You would watch him and you would cry laughing and you would think, 'I'm never going to see anyone like this human ever.' It's just amazing," he said.
The actor died in his California home Monday night.
Fallon closed his tribute with a nod to Williams' performance in The Dead Poet Society by standing on his desk and saying, "O Captain! My Captain! You will be missed."