(Reuters) - American Russell Henley, remarkably unflappable for a PGA Tour rookie, became the youngest ever champion at the Sony Open in Hawaii when he won his maiden title on the U.S. circuit by three shots on Sunday.

Aged just 23, the fresh-faced Web.com Tour graduate displayed ice-cool nerves as he stormed home with five consecutive birdies to fire a seven-under-par 63 on the tree-lined layout at Waialae Country Club.

Co-leader overnight with fellow rookie and good friend Scott Langley, Henley mixed eight birdies with a lone bogey to post a record 24-under total of 256 in the PGA Tour's first full-field event of the season.

South African Tim Clark, bidding for his second title on the circuit, signed off with birdies on the last four holes for a matching 63 to secure second place.

First-round leader Langley faded with three bogeys on the back nine, carding a 70 to finish in a tie for third at 17 under with fellow American Charles Howell III (66).

(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)