Tied with Davies heading to the 16th hole, Donald claimed the lead after hitting his approach shot from 252 yards to within 12 feet of the hole and sinking the eagle putt.
Donald sealed his first title since 2006 when his Welsh rival missed a chance to take it to a playoff by missing a 20-foot putt at the last.
Donald shot a final-round 5-under 67 to finish at 21-under 267. Davies finished with a 68, while Italian star Francesco Molinari also shot 68 and was three shots back.