Buchan. 30, scored back-to-back 9-point rides in the final heat to edge the American veteran by a score of 18.94-17.90.
"Every time I paddled out, I told myself that I only had to catch two waves," Buchan said. "That was my mantra through each heat and it worked out well for me."
Slater made a late charge in the finals with an inspired but right-handed barrel maneuver in the 4-to-6-foot curls, but Buchan built up enough points to ice the crown.
The victory was Buchan's second Association of Surfing Professionals championship. Slater remained in the lead in the ASP points total with 38,950 points. Australian Mick Fanning is in second with 37,900. The ASP Tour moves on to Trestles in Southern California Sept. 15-21.