Paul Casey labelled Collin Morikawa as "something special" after the American's US PGA Championship win.

Morikawa carded a six-under 64 in the final round at TPC Harding Park in San Francisco on Sunday to beat Casey (66) and Dustin Johnson (68) by two strokes.

The 23-year-old became the third-youngest player to win the US PGA since World War II, behind only Rory McIlroy and Jack Nicklaus.

Casey, who has 10 top-10 finishes in majors without a win, paid tribute to Morikawa.

"I played phenomenal golf and there's nothing I would change. I'm very, very happy with how I played, great attitude, stayed very calm and stayed in the present. It wasn't enough," the Englishman said.

"The glorious shots Collin hit like on 16 to make eagle, you have to tip your cap. When he popped up on Tour not that long ago, those guys who were paying attention like myself knew that this was something special, and he's proved it today.

"He's already sort of proved it but he's really stamped his authority of how good he is today.

"But I'm very, very happy with everything. I kind of got my mojo back now."

Morikawa produced some magic at the 16th hole, putting an incredible 293-yard tee shot to within seven feet for eagle.

Casey had just made birdie at the same hole and the 43-year-old hailed Morikawa for his shot.

"It's my tee shot on the 17th which was bothering me more," he said.

"Brilliant shot. I love the fact we've got drivable par-fours. You know I'm a big fan of the shorter hole. I love the fact that we're given an opportunity, and then a guy like Collin steps up and shows you what's possible on a drivable par-four. Nothing I can do except tip my hat. It was a phenomenal shot.

"I knew he made the putt because we could hear the small roar, the small clap and cheer when he holed it in. I think I was on the 18th tee at the time. Yeah, I was very focused on myself, so other than acknowledging what a shot he hit, I was trying to take care of business."