The actor’s family, including his wife Emma Heming-Willis and ex-wife Demi Moore, announced his condition on Instagram on Wednesday.
Aphasia is “impacting his cognitive abilities”, the statement said.
Willis, 67, is best known for playing John McClane in the Die Hard films, which made him a star.
“With much consideration Bruce is stepping away from the career that has meant so much to him,” his family wrote in a joint statement.
“To Bruce’s amazing supporters, as a family we wanted to share that our beloved Bruce has been experiencing some health issues and has recently been diagnosed with aphasia, which is impacting his cognitive abilities. As a result of this and with much consideration Bruce is stepping away from the career that has meant so much to him.
This is a really challenging time for our family and we are so appreciative of your continued love, compassion and support. We are moving through this as a strong family unit, and wanted to bring his fans in because we know how much he means to you, as you do to him.
As Bruce always says, “Live it up” and together we plan to do just that.
Emma, Demi, Rumer, Scout, Tallulah, Mabel, & Evelyn”
His acting career began in the early 1980s but he did not become a household name until later in that decade – first after starring opposite Cybill Shepherd in the ABC TV series Moonlighting and then in his 1988 performance as John McClane in the first Die Hard film.
Since then, his films including The Sixth Sense, Armageddon and Pulp Fiction have grossed more than $5bn worldwide, according to Variety. He’s been nominated for five Golden Globes, winning one for Moonlighting, and three Emmys, winning two.
Several actors and other stars offered their condolences to Willis and his family following the news.
“Grace and guts! Love to you all!” actress Jamie Lee Curtis wrote in response to Demi Moore’s post.