The Pole, who has 13 career titles to her name, has struggled to make too much of an impact this season, but was convincing in disposing of Williams at the WTA Premier hard court event in Montreal.
Third seed Radwanska won four games in a row in the first to storm into a 4-1 lead after Williams had claimed the opener.
Williams battled hard to break back in a tight game six, but was unable to complete a comeback after seeing two break points come and go at 4-3 down.
The American struggled on her first service throughout and that fact was emphasised by a second set in which she mustered only a 42 per cent success rate.
Combined with an inability to win points on her second serve - Williams won just one out of 12 in the second set - it proved to be a recipe for disaster.
Radwanska claimed the opening game of the second on Williams' serve to put the former world number one on the back foot from the off.
Williams broke back to level matters at 2-2, but Radwanska won the next game to love as the seven-time grand slam winner's chances of coming away with the win grew ever slimmer.
As it turned out, Williams would not win another game, leaving Radwanska to emphatically serve for the title.