The South Korean became the youngest ever champion of the tournament.
An impressive final round of three-under-par in Florida at Ponte Vedra Beach saw Kim finish 10 under.
He ended three shots ahead of Englishman Poulter, who was tied for the lead at one stage.
Louis Oosthuizen tied him for second place, while Rafa Cabrera Bello and Kyle Stanley finished tied for fourth on six under.
The stunning achievement by 21-year-old Kim, though, saw him follow in the footsteps of Tiger Woods, Sergio Garcia and Jordan Spieth.
All four have won twice on the PGA Tour before the age of 22 in the last 25 years.
During Kim’s final round, the real advantage over Poulter came when the South Korean birdied the seventh and ninth holes.
After 39 consecutive holes without dropping a shot, Poulter then finally bogeyed on the 12th.
He then repeated the feat on the 18th as Kim kept his focus on the back nine on to secure the biggest win of his career so far.
Poulter’s near miss was made all the more impressive by his serious foot injury which plagued him in 2016.
After clinching the Players Championship, Kim said: “I feel like I’m still dreaming. I never expected I was going to win this tournament.
“I wasn’t doing that well at the beginning of this year, but I’m so excited I can be the champion.
“It was a great experience for me that I won one of the championships last year and today because of that experience I could be stable and focus on myself and not try to think about other scores – that really helped me to be stable.”
Meanwhile, Poulter said: “It’s been a great week, to be in a position a couple of weeks ago where I wasn’t here to now at tied two – it’s been a great week.
“It has been tough – a tough 18 months. Today I felt a couple of putts slipped by. I played well under pressure barring that second shot at the last.
“I’ve had the kids up here and had some fun on the beach, it’s taken my mind off things. I’m enjoying my golf and hopefully this is a stepping stone for this year.”
