The Texan, 22, won his eighth title on the PGA Tour in style, finishing with birdies at the last three holes to triumph by three strokes.
It was the first time he had won a title in his home state.
Meanwhile, Englishman Chris Wood climbed 32 places to 22nd thanks to his win in the European PGA Championship at Wentworth on Sunday.
Official World Golf Ranking top 20:
1. Jason Day (AUS) 13.40 pts
2. Jordan Spieth (USA) 11.60
3. Rory McIlroy (NIR) 9.66
4. Bubba Watson (USA) 7.77
5. Rickie Fowler (USA) 7.41
6. Henrik Stenson (SWE) 7.10
7. Adam Scott (AUS) 6.85
8. Dustin Johnson (USA) 6.60
9. Danny Willett (ENG) 6.55
10. Justin Rose (ENG) 5.90
11. Branden Grace (RSA) 5.18
12. Patrick Reed (USA) 5.06 (+1)
13. Sergio Garcia (ESP) 5.00 (-1)
14. Louis Oosthuizen (RSA) 4.76
15. Hideki Matsuyama (JPN) 4.60
16. Brooks Koepka (USA) 4.18
17. Brandt Snedeker (USA) 3.98
18. Matt Kuchar (USA) 3.91 (+2)
19. Phil Mickelson (USA) 3.87 (-1)
20. Zach Johnson (USA) 3.83 (-1)
Selected other
22. Chris Wood (ENG) 3.61 (+32)