Donald would have jumped from No. 3 to No. 1 with a victory, but lost to Brandt Snedeker on the second hole of a playoff in The Heritage at Hilton Head Island.
After waiting out a lightning delay on the final hole, Westwood finished off 3-under-par 69 for a three-stroke victory over Thailand's Thongchai Jaidee, who closed with 65.
The English star, ranked second behind Martin Kaymer entering the week, won on his 38th birthday.
Asked what he wanted for his birthday, Westwood replied, "Something silver and shiny."
As he stepped off the 18th green a portion of the gallery sang "Happy Birthday."
Westwood finished at 19-under 269 in the Asian Tour event at Royale Jakarta. He took a five-stroke lead into the final day after opening with rounds of 68, 66 and 66.
"It was quite a day, really," Westwood said. "I knew it wasn't going to be easy with a five-shot lead. I'm experienced enough to know that patience is the key."
CHAMPIONS TOUR: In Savannah, Ga., Kenny Perry missed a 14-inch par putt to extend the match and David Eger and Mark McNulty won the better-ball Liberty Mutual Legends of Golf on the second playoff hole against Perry and Scott Hoch.
Eger and McNulty, who started four shots off the lead in the 54-hole event at The Club at Savannah Harbor, shot 11-under 61 in the final round for 27-under 189. Perry and Hoch shot 64 to get into the playoff.
Second-round co-leaders Wayne Levi and Keith Fergus finished with 66 for 26-under 190, tied for third with Russ Cochran and Mark Weibe (60),
Augusta native Larry Mize and Hal Sutton shot 64 and tied for eighth at 191. Former Augustan Allen Doyle and Bruce Vaughan had 65 and tied for 18th place at 195.
CHINA OPEN: In Chengdu, Nicolas Colsaerts shot 6-under-par 66 and set a tournament record, finishing at 24-under to win by four shots over a group of four players.
Pablo Martin of Spain (63), Peter Lawrie of Ireland (68), Soren Kjeldsen of Denmark (66) and Danny Lee of New Zealand (65) were next/
Former Augusta State All-American Oliver Wilson closed with 70 to finish at 280 and tie for 59th place.