Na, 35, who led by two shots overnight, birdied four of the first eight holes at Colonial Country Club and finished on 13 under to take his third PGA Tour title.
Tony Finau carded a final-round 68 to seal second place, one shot ahead of another American, Andrew Putnam, who closed with a 66.
Na swept into contention by carding a second-round 62, one shot off the course record, and overshadowed local favourite Jordan Spieth.
Spieth, without a win since the 2017 Open at Royal Birkdale, shot a two-over-par 72 in the final round to finish tied in eighth place, alongside Scotland’s Russell Knox and England’s Tyrell Hatton, eight shots behind Na.
Defending champion Justin Rose finished tied in 58th place on five over after a final round of 70 followed a 74, 67 and 74.