World number five Rose shot a round of a five-under par 66 to hold off Westwood after he moved into a one-shot lead with a birdie on the opening hole and secured victory by holing a 20-foot birdie putt on the 17th.
The prize cheque is the biggest in Rose's 14-year professional career and $560,000 more than his winnings in capturing last year's BMW Championship.
World number four Westwood earned a cheque for $1 million for his efforts over the three days in the eight-player event that also included world No.1 Rory McIlroy and Tiger Woods.
Just as in the Ryder Cup against Phil Mickelson, Rose found the 17th green to his liking.
Never behind in his match after starting with a first hole birdie, Rose went two ahead with a birdie at 10 and while Westwood birdied the 16th to halve the deficit, Rose holed a 20-foot putt for birdie at the 17th.
It was a similar length to the one he sunk at Medinah to turn around his match against Mickelson.
A day earlier, Rose had chipped in at the 17th to deny Woods a place in the final.
"The 17th green has been really good to me this week as I holed my second shot there yesterday and, of course, the 17th at Medinah turned around my match against Phil," Rose said.
"You have to be pleased to go through this whole week after winning all my five matches. To not get beaten at all in the group stage and then to win the two finals is a great feeling.
"My golf is just so consistent at the moment and the main thing is that I just don't have any skeletons in the closet, I don't have that loose shot that is plaguing me all the time."
Westwood, who was 22-under par for his five rounds, said Rose's putting had been the difference.
"We both played well with just the slight difference on the greens," he said. "Justin rolled some 20 footers, another crucial one at 17 while the longest putt I made was eight or nine feet on 16.
"I knew before today the he was rolling it well on the greens. He showed that against Phil Mickelson at the Ryder Cup."