Industry sources estimated that the French house spent about $10 million (£6.2m) advertising in the US and $7 million (£4.3m) for his "services", reports WWD.
The Hollywood star's ad campaign will roll out in mid-October, which will feature a commercial and print ad.
Pitts is the first male to promote Chanel No.5, an iconic perfume for women. Its list of female spokesmodels include Nicole Kidman, Audrey Tautou and Vanessa Paradis.
Chanel's president for fragrance and beauty Andrea d'Avack said: "To keep a legend fresh, you always have to change its point of view. It is the first time we've had a man speaking about a women's fragrance.
"We think very much that the perfume is a seduction between a man, a woman and the perfume. No.5 is our leading fragrance, and we are willing to make the investment to keep it on that level."
Pitt can also be seen next in the crime thriller Killing Them Softly, out on November 30 in the US. It is now playing in the UK.
Watch the trailer for Killing Them Softly below: