The 67-year-old was diagnosed with aphasia, which causes speech difficulties, in the spring of last year, but this has advanced and he has been given a more specific diagnosis, according to his family.
They expressed their "deepest gratitude for the incredible outpouring of love".
the 1980s and 1990s as the star of blockbuster blockbusters such as Die Hard, The Sixth Sense, Armageddon, and Pulp Fiction.
He's also been nominated for five Golden Globes, one of which he won for Moonlighting, and three Emmys, two of which he won.
Willis' family said last year that he will stop performing since his aphasia was impairing his cognitive ability.
The word dementia describes the impact of a diseased brain on our memory, language and thinking skill.
Bruce Willis' diagnosis of fronto-temporal dementia is relative rare.
It is also unusual as it largely affects people in midlife, whereas most other forms are found in old age.
There is still no cure or even a way of slowing the disease down so symptoms continue to get worse.