The Oscar-winning American actress will star as Dr. Grace Trevelyan Grey who adopts Christian when his birth mother dies.
She joins a star-studded cast which includes Jamie Dornan as Christian, Dakota Johnson as Anastasia Steele and singer Rita Ora as Christian's sister.
As the cast members names are revealed, filming is underway in Vancouver.
Jamie and Dakota were spotted shooting one of the book's first scenes which sees the pair meet for the first time for an interview in a coffee shop.
The RIP singer confirmed she would be playing Christian's sister Mia on Twitter on Monday night.
"It's official!," the excited 23-year-old wrote of her first major film role. "I've been cast in @FiftyShades of Grey as Christian's sister Mia."
Marcia, who starred in Miller's Crossing and Into The Wild, has also recently joined the cast of Woody Allen's new film Magic in the Moonlight.
The 54-year-old won an Academy Award for her supporting role in 2011 film Pollock and was nominated in the same category in 2004 in Mystic River.
It was announced in June that British director Sam Taylor-Wood would helm the highly-anticipated screen adaptation of E. L. James' hit book.
Production was pushed back by a month after Sons of Anarchy actor Charlie Hunnan dropped out of film, weeks after he was cast as the leading man.
Fifty Shades of Grey will be released on Valentine's Day weekend in 2015.