Johnson led by three strokes going into the final round only to have Poulter catch him early Sunday. But Johnson had three birdies and eagled the par-4 16th in finishing with a 6-under 66 and winning by three strokes with a tournament-record 24-under 264.
Sunday marked the eighth PGA Tour win for Johnson who collected more than $1.3 million along with the title.
Poulter also had a 66 Sunday and ended up 21 under while Graeme McDowell, also with a final-round 66, was third at 20-under. Poulter took home about $865,000 and McDowell nearly $500,000
Johnson started the day up by three over Poulter but three-putted the first hole for a bogey and then had six pars. Poulter caught Johnson with birdies at Nos. 1 and 2 and led after a fifth-hole birdie. Johnson matched Poulter's birdie at 8 and pulled even with a birdie at the ninth hole.
Poulter was up one after a 12th-hole birdie only to have the tournament quickly turn. Johnson birdied Nos. 13 and 14 and Poulter had a bogey at the par-4 15th that returned the lead to Johnson. A chip-in eagle 2 at the 16th pushed the Johnson lead to two and as 17th-hole birdie sealed the win.