The title was the Fijian's 32nd on the PGA Tour, most by a foreign-born player.
Singh finished the event at the Firestone Country Club in Akron at 20-under 270.
Singh parred the final five holes, managing to hold off his pursuers. Appleby birdied 16 and 17 but parred the 18th.
"Except of the second day, when I didn't drive the ball that well, I struck the ball well this week," Singh said. "I putted quite decent, except today. The short ones got kind of shaky there. I managed to get it around though."
Singh won $1.3 million.