It was the first win of the season for Radwanska but provides a timely jolt of confidence going into Flushing Meadows and the year's final grand slam with 11 of her 14 career titles having come on hard courts.
Williams, a twice U.S. Open champion, will also be buoyed by her performance in Montreal with a projected return on Monday to the world rankings top 20 for the first time since March 2013.
The tournament also pushed Williams past the $30 million in career earnings, joining an elite club that includes her sister Serena and Maria Sharapova.
After a grinding three-set win over top ranked sister Serena in Saturday's semi-finals, the 34-year-old American appeared to have little left against an energetic and focused opponent almost 10 years her junior.
Radwanska took control early on a sunny centre court, breaking Williams twice to race in front 4-1.
Williams would break back but the Pole would not buckle and easily took the opening set.
The world number five opened the second set with another deflating break to quickly put the pressure on Williams and then broke the dejected American again to go up 5-2 before holding serve and finishing off the match with an emphatic ace.
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