“While this is painful, it is a relief to finally have a clear diagnosis,” the family said of the 67-year-old actor in a statement.

“FTD is a cruel disease that many of us have never heard of and can strike anyone.”

Last March, Willis’s family said his aphasia had affected his cognitive abilities. The condition causes loss of the ability to understand or express speech.

In Thursday’s statement, his family said communication challenges were just one symptom of frontotemporal dementia.

“Today there are no treatments for the disease, a reality that we hope can change in the years ahead,” the statement read.

“As Bruce’s condition advances, we hope that any media attention can be focused on shining a light on this disease that needs far more awareness and research.”

The statement was posted on the website for The Association for Frontotemporal Degeneration and signed by Willis’s wife, Emma Heming Willis, his ex-wife Demi Moore, and his five children, Rumer, Scout, Tallulah, Mabel and Evelyn.

Over a four-decade career, Willis’ movies had earned more than $US5 billion ($A7.2 billion) at the worldwide box office. While beloved for hits like Die Hard and The Sixth Sense, the prolific actor had in recent years primarily featured in direct-to-video thrillers.

-with AAP