Both players finished 72 holes with scores of 15-under-par 273, with Imada two-putting for birdie on the 72nd hole.
Johnson shot 5-under 67 to force the playoff.
At the par-five 18th, which was the playoff hole, Imada found the left rough off the tee, and Johnson hit a big drive down the fairway.
Imada tried to force a shot from the rough onto the green, but his approach went left and into the water.
Johnson calmly knocked his second shot onto the putting surface, and went on to win at the TPC Sugarloaf course for the second time.
"I'm not sure what it is, but for whatever reason, I've had success in this state," Johnson said.
The victory was worth $972,000.