Two strokes behind Pavin after 10 holes in the final round, the 50-year-old Couples played the final eight in 7 under, making an eagle -- his second of the round -- and five birdies.
The former University of Houston star finished at 17-under 199 on The Woodlands Country Club course and earned $255,000 for his fourth Champions Tour victory of the year.
Mark Wiebe (71) was second, Dan Forsman (66) and Brad Bryant (66) followed at 9 under, and Pavin (74) and John Cook (67) tied for fifth at 8 under.
Couples birdied Nos. 11 and 12 to tie Pavin, then made a long eagle putt on the par-5 13th. After Couples' eagle, Pavin made a bogey, missing a 7-foot putt.
Couples moved two more shots ahead on the next hole with another birdie and Pavin's bogey.
Couples also birdied Nos. 16 and 18.
Bernhard Langer, 535 points ahead of second-place Couples in the Charles Schwab Cup standings, shot a 66 to tie for 13th at 3 under. Kenny Perry tied for 35th in his first start on the 50-and-over tour, closing with a 70 to finish at 1 over.