Universal Pictures and Focus Features confirmed Marcel's appointment on Monday (October 8).
EJ James's hit erotic novel, on which the film is based, revolves around the relationship between 27-year-old billionaire Christian Grey and college student Anastasia Steele.
The rights to the book and its sequels Fifty Shades Darker and Fifty Shades Freed were acquired by Universal and Focus in March. The trilogy has sold more than 32 million copies, and has been translated into 45 languages worldwide.
Marcel's previous credits include the 2011 Black List script Saving Mr. Banks.
The movie - which recalls Walt Disney's lengthy pursuit of the film rights to P.L. Travers's book Mary Poppins - is currently in production at Walt Disney Studios. Emma Thompson has been cast as Travers, with Tom Hanks and Colin Farrell also starring.
Marcel was also a writer and executive producer on Fox's short-lived dinosaur drama Terra Nova.
Fifty Shades of Grey co-producer Michael De Luca said: "Kelly's work demonstrates her flawless structural technique and passionate commitment to emotion, humour and depth of character, which is particularly visible in the celebrated screenplay for the upcoming Saving Mr. Banks."
"We were all taken with the depth and passion of Kelly's engagement with the characters and world EL James has created, and we knew she was the right person to augment our Fifty Shades family," added De Luca's producing partner Dana Brunetti.
Photo gallery - Fifty Shades of Grey - casting rumours: