By comparison, Potter author J.K. Rowling's first novel for adults, "A Casual Vacancy", took three weeks to reach 100,000 sales and is currently fourth on the top 20 list, the French weekly Livres-Hebdo reported Thursday.
The "mommy porn" book's success comes despite devastating criticism from France's literary establishment.
"The book is as close to literature as Whiskas cat food is to gastronomy," L'Express magazine wrote, while the French edition of Cosmopolitan magazine called it, "insipid" and "dull".
The novel by British author E.L. James, which follows the relationship of college graduate Anastasia Steele and business magnate Christian Grey, was first published in 2011.
The two other books in the trilogy, "Fifty Shades Darker", and "Fifty Shades Freed", will be released in France early next year.