The Spaniard took the first set 6-4. The set had gone to serve until the tenth game when Janowicz handed Ferrer a set point by serving a double fault.
The Pole started the second brightly, breaking the world number five in the third game. Ferrer broke back straight away, before opening up a commanding 5-2 lead.
The 21-year-old held his serve to take the set to 5-3 but couldn't get another break against the Ferrer serve.
The win gives Ferrer his seventh title of the year and his first Masters 1000 win.