Johnson carded a closing 66 to finish at 24-under-par 264 at Sheshan International with Poulter also shooting 66 for second place on 21-under. Northern Irishman Graeme McDowell was another shot back in third after also carding 66.
"It's the biggest win I've had so far in my career," Johnson told reporters.
The 29-year-old American started the final round with a three-shot lead over Poulter but surrendered it after an opening bogey as the Englishman started birdie-birdie.
Undeterred, Johnson picked up five birdies and an eagle from the eighth to collect the $1.53 million cheque for his eighth victory on the PGA Tour.
"I knew if I stuck to my game plan it would come," Johnson said. "I was swinging well and putting well.
"I wasn't having too much fun at the start, especially when Graeme and Ian were birdieing every hole it seemed like.
"They put a lot of pressure on me but I'm really proud of the way I handled myself. I thought I controlled my nerves pretty well today."
Johnson is renowned for his long hitting but he credited his win to some neat short game played under pressure on the back nine.
"The biggest putt I made was on 14 (from 20 feet) when Ian was already in for birdie - and the pitch on 16 was huge," he said.
"I was talking to my brother (caddie) right before I hit it. I said 'I think I'm going to make this one. I hit it perfect'. After (that) I was feeling pretty good where I was at."
Johnson, who blew a chance to win the 2010 US PGA Championship when he incurred a two-stroke penalty for a rules violation at the final hole, said he hoped the significant victory in China would lead to him winning a major.
With his long hitting and deft touch around the greens, the U.S. Masters appears a good bet.
"Majors are always the step," he said. "If I play like I did this week I'm going to win one for sure. I really like Augusta.
"That would be my favourite one. I grew up right down the street from Augusta and I really love playing that golf course."
McDowell only had praise for Johnson, who finished tied 13th at the Masters earlier this year.
"He's just a quality, talented, very athletic classy player," the former U.S. Open champion said. "He's got a great wedge game to go with an outrageously good driving game.
"Yeah, he makes mistakes, but when you've got a game as good as him, you can get away with a few mistakes."
McDowell is second behind Henrik Stenson on the European Tour's Race to Dubai standings, with Poulter moving up to fourth after he put up a brave defence of the title he won last year.
Poulter, though, was disappointed that he could not have closed the gap further on Swede Stenson, who finished tied 31st, with only two events remaining in Turkey and Dubai in the European Tour's Final Series.
"I guess it was a good defence," the Ryder Cup winner said.
"I'm a little disappointed not to put my hands back on the trophy but Dustin kind of finished the job.
"I'm in great form. The work I've done leading up this run of tournaments has really worked and I couldn't be hitting it any better right now."
Spain's Sergio Garcia fired a brilliant 63 to snatch fourth on 18-under, two ahead of England's Justin Rose (68) in fifth.
Former world number one Rory McIlory was another European Ryder Cup winner in the top 10 after he closed with a 69 to finish tied sixth with Canada's Graham Delaet.
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