The 56-year-old actor - who was nominated for an Academy Award for Best Supporting Actor for 'The Last Samurai' - revealed that he has been forced to delay his return to Broadway's 'The King and I' while he battles the disease.
Ken has undergone surgery and is currently recuperating in hospital in his native Japan, according to the Kyodo News service.
Ken Watanabe has revealed he is battling stomach cancer
In a statement, he said: “I would like to ask for your understanding that the start of my performance in New York will be a little delayed."
Ken has starred in Christopher Nolan's 'Batman Begins' and 'Inception', along with movies including 'Godzilla' and 'Letters From Iwo Jima'.
He first appeared on Broadway last year in the revival of 'The King and I' and was due to return to the Rodgers and Hammerstein musical from March 1-April 17.
Ken was previously diagnosed with acute myeloid leukaemia in 1989 and after he defeated it, it returned in 1994. He went into remission two years later.
Explaining his reluctance to talk about his illness previously, he said: "As an actor, I didn't want to talk about it. If I create a strong guy, everyone couldn't see the strong guy. This is inside of me."
- By Zoe Shenton