Horschel overcame a pair of lengthy rain delays and used the late birdie to finish one stroke ahead D.A. Points at 20-under 268.
The Florida native reeled off six straight birdies from the seventh to the 12th holes.
Points -- who beat Horschel by one stroke last month at the Houston Open -- fired a 7-under 65, settling for second place at 19-under.
Kyle Stanley (67) finished third at 17-under 271 was Kyle Stanley.
Third-round leader Lucas Glover stumbled to a 1-under 71 and ended in a share for fourth at 15-under with Bobby Gates (66).