Darrell Crosby, 59, of Jefferson died Monday at Pen Bay Medical Center in Rockport.
At the time of his death, Crosby had served about half of an 18-year sentence for unlawful sexual contact. He was convicted of molesting a little girl over several years, starting when she was 4 years old.
Crosby was battling cancer when he was sentenced. The prison hasn't released any details about what caused his death.