Koepka finished on 20-under 264 for the $1.8 million tournament at Phoenix Country Club in Miyazaki with compatriot Xander Schauffele (71), Thai Prayad Marksaeng (67) and South Korean Lee Sang-hee (72) well beaten in a share of second.
Hideki Matsuyama looked like he might be launching a Sunday charge when he fired a hole-in-one at the par-three third hole but a double-bogey just after the turn effectively ended his chances of a second Dunlop Phoenix title after his 2014 win.
Japan's world number four revived his round with three birdies on the back nine but another bogey at the 17th saw him drop out of a share of second and he finished fifth on 10-under after a 69.
Koepka's maiden major triumph in June came on the back of a late surge at Erin Hills but there was no need for anything spectacular on the back nine on Sunday after he started the day with a four-stroke lead and made a sizzling start.
Birdies on his first two holes and another at the par-five seventh took the 27-year-old well clear of the field at the turn, and bogeys at the 10th and 17th were offset by a shot picked up at the 13th and his eagle-three on the 18th.
The former European Tour regular had made retaining a title one of his goals for the year and he can now head off to Tiger Woods's Hero World Challenge in the Bahamas with that mission accomplished.
Matsuyama, who finished second behind Koepka at the U.S. Open, will be the defending champion at the Nov. 30-Dec. 3 tournament in Albany, where Woods will make his comeback after nine months away from competitive golf.
(Reporting by Nick Mulvenney in Sydney; Editing by John O'Brien)
