Woods hurt his back making an awkward slash at the ball and falling backward while trying to play off the lip of a bunker.
"It happened on the second hole when I hit my second shot, I fell back into the bunker," Woods said. "I just jarred it, and it's been spasming ever since."
Woods made several poor shots during his round before leaving the course on a cart with caddie Joe LaCava after hitting a tee shot on the ninth hole. LaCava told PGAtour.com that Woods' back had been hurting him all day.
LaCava drove Woods straight to the parking lot, where the golfer appeared to be in pain while taking off his shoes in front of his car.
"It's just the whole lower back," Woods told reporters. "I don't know what happened."
Woods entered the final round well down the leader board before finishing his partial final round at three over par. Woods is the defending Bridgestone Invitational champion.
Woods, 38, entered this week's tournament looking to get his game back on track. He sat out for two months months after having surgery on his back for a pinched nerve on March 31. Woods missed the Masters and the U.S. Open before making his return at the Quicken Loans National on June 26. He has played in three tournaments since his return.
Woods told reporters Wednesday that his return from surgery has been more challenging than his return from previous injuries.
"There is no comparison between a knee and a back," he said. "This thing is just way different. It's way more debilitating than I thought."
Woods went into this week's tournament at Firestone Country Club in Akron, Ohio, looking to improve on all areas of his game.
"Everything needs to get a little bit better," he said.