Jordan Spieth has won his second major of the year by clinching the US Open at Chambers Bay.

The 21-year-old secured his victory with a one-under-par round of 69 on the final day to finish five-under for the tournament, but he was at the mercy of Dustin Johnson at the 18th hole.

Johnson had the chance to win the second major of 2015 with an eagle putt, but his stroke went wide and four feet past the hole, although he did have a second opportunity to send the tournament into a playoff with his compatriot.

However, Johnson lost his composure at the vital moment as he missed from four feet, handing the title to Spieth.

The 21-year-old began the day on four-under, tied with Branden Grace, Jason Day and Johnson, but he fell back to three-under when he bogeyed the opening hole.

He started to find his form on the eighth with a birdie and began to pull ahead with further birdies on the back nine, including one excellent putt on the 16th, which moved him to six-under.

A poor tee shot on the 17th put him in trouble and Spieth lost his composure on the green as he missed his bogey putt, and dropped back to four-under tied with Johnson and Louis Oosthuzien.

The South African put the pressure on the leaders with a final round of three-under 67, taking just 29 shots on the back nine.

Spieth responded with a solid drive on the 18th and made an even better stroke to have a chance at an eagle putt. The 21-year-old's first putt went centimetres wide, but he was left with a tap-in to move to five-under.

The American watched his compatriot move within 12 feet of the hole on the 18th, but unlike Spieth, Johnson was unable to hold his nerve as the 21-year-old won his second major of the year.

Spieth is the youngest winner of the competition since 1923, and is the fourth youngest player to win two major championships.