PHOENIX, 1 Feb 2015 (AFP) - Koepka fired a bogey-free 66, five-under par, to finish 72 holes on 15-under 269, one stroke in front of Watson, fellow American Ryan Palmer and Japan's 22-year-old Matsuyama, who missed an 18-foot birdie putt on the final hole to ensure 24-year-old Koepka's triumph.
"It feels amazing," Koepka said. "I had quite a few scares but it was a fun day. Me and Hideki will be battling for quite a while. I hope we're doing it for a few more years."
Koepka, ranked 33rd, shared fourth at last year's US Open and won his first European Tour title last November at the Turkish Airlines Open.
"The last few weks I've put in a lot of hard work, changed my putting stroke completely," Koepka said. "It paid off."
Scotsman Laird, the 54-hole leader, stumbled at the finish with a bogey at 17 and a closing double-bogey to share fifth with Spanish amateur Jon Rahm on 272.
Watson made an early charge with three birdies in a row starting at the second hole, then closed the front nine with a birdie and added birdies at the par-3 12th and par-4 13th. He posted the target score in the clubhouse and it nearly paid off.
"A couple shots here and there," Watson said. "On 17, 3-putted but I had an 80 footer, then the par-5 (15th) I didn't hit my tee shot good enough. It was a great day. It was a good challenge. I'll accept it at the end of the week."
Laird, Matsuyama and Koepka battled in the final group.
Koepka birdied six and seven, the 13th as well and sank a 50-foot eagle putt at par-5 15th.
Laird, who opened with a birdie, followed with 13 pars until two-putting from 28 feet for birdie at 15 to top Koepka and match him for the lead at 15-under.
Matsuyama opened by making an eagle out of a sand-filled fairway divot, birdied the third, fourth and 13th but stumbled by missing a six-foot par putt at 14 to stand one off the lead.
At 17, Laird missed an eight-foot par putt to join Matsuyama one off Koepka's pace as the trio headed for the 18th tee.
Laird found water with his first shot to doom his title bid and Matsuyama found a fairway bunker but blasted his approach 18 feet from the pin.
Koepka, who never missed a back-nine green over the weekend, put his approach 20 feet from the cup and rolled his birdie putt inches from the cup. After Matsuyama missed, Koepka tapped in for the victory.
Scores after Sunday's final round of the $6.3 million US PGA Phoenix Open (USA unless noted, par 71):
269 - Brooks Koepka 71-68-64-66
270 - Bubba Watson 65-71-69-65, Ryan Palmer 64-72-68-66, Hideki Matsuyama (JPN) 69-71-63-67
272 - Jon Rahm (ESP) 70-68-66-68, Martin Laird (SCO) 66-66-68-72
273 - Freddie Jacobson (SWE) 68-73-68-64, Jordan Spieth 70-68-70-65, Graham DeLaet (CAN) 67-70-69-67
274 - Brandt Snedeker 70-68-70-66, Brian Stuard 72-68-67-67, Daniel Berger 65-69-71-69, Robert Streb 66-70-69-69, Zach Johnson 66-70-67-71
275 - Angela Cabrera (ARG) 67-69-69-70, Russell Knox (SCO) 69-71-65-70
276 - Aaron Baddeley (AUS) 68-71-71-66, Keegan Bradley 65-73-71-67, Andrew Svoboda 70-70-68-68, Ryan Moore 69-67-69-71, Justin Thomas 67-68-69-72
277 - Tony Finau 72-68-70-67, KJ Choi (KOR) 68-69-72-68, Rory Sabbatini (RSA) 68-71-67-71, Francesco Molinari (ITA) 70-71-64-72
278 - Brendan Steele 71-67-71-69, Brendon de Jonge (ZIM) 67-71-72-68, Kevin Na 73-69-66-70, Pat Perez 70-69-68-71