Jordan Spieth
Spieth seals second major of 2015.Getty

Jordan Spieth has become the youngest US Open champion since 1923 after edging past Dustin Johnson in a thrilling conclusion at Chambers Bay.

Spieth, 21, sealed his second career major following his Masters triumph in April, carding a one-under 69 on the final day and getting to the clubhouse under par with victory in his sights.

A poor double bogey at the 17th appeared to leave the world no. 2's hopes of victory in jeopardy but the Texan recovered in splendid fashion, hitting two sublime shots on the final hole to set up a simple two-putt birdie.

Johnson had the opportunity to draw level after Spieth's nightmare on the 17th where he found a birdie but agonisingly three-putted from 12 feet on the final hole.

World no. 1 Rory McIlroy (66) briefly threatened to spark into life on the final day, hitting a superb 72-foot birdie on the 13th, but ended the day level par.

Adam Scott and Louis Oosthuizen also provided late charges on an exhilarating final day, with the South African sinking six birdies in his last seven holes to tie with Johnson in second place.

But Spieth kept his cool to claim his second major of the year.

"I'm in shock but I feel for Dustin," said Spieth. "It's cool to be able to have two legs of the grand slam now, and to conquer golf's hardest test - the US Open - is conquering the hardest layout in all of golf.

"I didn't have my best ball-striking at all and really grinded over those four or five-footers - that was the difference."

Spieth began the final day tied with Johnson, Jason Day and Branden Grace. Despite bogeying on his first hole he strung together two birdies and 12 pars before the crucial 16th hole, where the youngster moved three clear after Grace drove his tee shot 50 yards out of bounds.

The South African settled for a double bogey after that to drop to three under, while Spieth moved six under with a fine 28-foot birdie.

Johnson and Spieth remained level until the par-three 17th where the latter was able to hold onto his advantage despite a double bogey, keeping his nerve on the final hole.