The 27-year-old star - who recently refused to move back to Los Angeles from New York so her family can offer her more support in her life - took to her Twitter account to reveal the condition is making it hard for her to remain in a good shape.
She tweeted: "I have an eatingdisorder so I have a hard time staying thin."
The `Easy A` actress recently described herself as "looking pudgy", and in February she announced plans to try to shed 21 pounds because she was unhappy with her size.
The brunette star - who was recently forced to move out of her New York apartment, after being threatened with eviction - explained: "I moved to New York City and I love it! I lost 4lbs since I moved. I`m 121lbs - my goal is 100lbs. I gain weight quickly so I need to work out constantly."
Amanda`s eatingdisorder revelation comes after she was involved in a series of legal troubles earlier this year, and in December she settled two hit-and-run cases after reaching a civil compromise with her victims, saving herself from a potential 12 months in jail.