Williams completely overwhelmed her unseeded opponent to cruise to a one-sided victory in just over an hour and claim the Rogers Cup for the third time.
It was the world number one's eighth title this year and put her outright fifth on the all-time list of tournament winners after she had been tied with Monica Seles.
Only Martina Navratilova, Chris Evert, Steffi Graf and Lindsay Davenport have won more WTA singles titles than Williams.
"There's a lot of pressure and nothing is guaranteed but I'm happy to do well," Williams said during the on court trophy presentation ceremony.
Cirstea had defied the odds just to make her third WTA final. The 23-year-old beat two former world number ones and two grand slam champions to reach the decider but found Williams too much to overcome.
"I know she is capable of winning big matches," Williams said. "She's had some big upsets in this tournament and I didn't want to be another casualty."
Williams got off to a fast start, breaking the Romanian's first two service games to jump out to a 3-0 lead.
Cirstea managed to break back in the next game then held serve to pull back to 3-2, but Williams immediately raised her level again and reeled off the next nine games in a row to seal victory.
Williams broke Cirstea to lead 5-2 when the Romanian hit a backhand long then clinched the set with a wide, slicing serve.
The American broke Cirstea's opening service game in the second set with a gorgeous drop shot while the Romanian struggled just to get a read on her opponent's serve.
"Thanks to my team for not running away," she said. "I know I'm not easy".
(Reporting by Matt Cronin; Editing by Julian Linden)
