In the final round, Koepka shot 67, three under par, giving him a total of nine under par for the event – two strokes better than compatriot Scottie Scheffler and Norwegian Viktor Hovland tied for second place.
Koepka plays on the controversial LIV Tour, becoming the first player from the controversial tour to win a major title.
The Swedes missed the "cut"
The victory was something of a revenge for Koepka, who led after both the second and third rounds of the US Masters, but fell through on the final round with six bogeis – and finished four strokes behind winner Jon Rahm in the first major of the year.
Swedes Alex Norén and David Lingmerth finished playing on Friday when they did not make the cut.