Cirstea had beaten top-10 trio Li Na, Petra Kvitova and Caroline Wozniacki en route to the final but she was powerless in the face of Williams.
The American roared to a 3-0 lead before Cirstea hit back for 3-2 but that was as good as it got for the Romanian.
Top seed Williams reeled off the next nine games to complete a 6-2 6-0 victory in 65 minutes.
It was Williams' third Rogers Cup title, having won in 2001 and 2011, and she is now in ninth on the all-time WTA title list.
Only eight players have more career WTA singles titles than Williams: Martina Navratilova (167), Chris Evert (154), Steffi Graf (107), Margaret Court (92), Evonne Goolagong-Cawley (68), Billie Jean King (67), Lindsay Davenport (55) and Virginia Wade (55).
Williams consoled Cirstea who was visibly emotional after being dismantled by her opponent, saying: "You're so young, I wish I was 23, don't be sad!
"You've beaten so many top players this week and you're really such a good player."
She added: "Sorana's one of the nicest girls on tour and has such a bright future."