11:00 AM PT -- A rep for Russell tells TMZ ... he was hospitalized for dehydration. He was kept overnight for observation, but they expect him to check out today.

11:15 AM PT -- Russell just responded himself, saying he was taken to the hospital last night for dehydration and joked his wife likes to remind him that he doesn't drink enough. He says he's on his way home.

Bill also playfully denied having any heart issues, saying ... "my friends know I don't have a heart to give me trouble."

NBA Hall of Famer Bill Russell is in the hospital in Washington state dealing with some apparent heart issues ... TMZ has learned.

Law enforcement sources tell us ... an ambulance was called to Russell's home in the Seattle area Friday night, and the 84-year-old Celtics legend was rushed to a nearby hospital.

We're told Bill was having heart trouble and shortness of breath ... and is still hospitalized. We've reached out to get an update on his condition.

The 11-time NBA champion center was also hospitalized in 2014 after collapsing at a speaking gig in Lake Tahoe, but recovered quickly.

Bill's considered one of the greatest basketball players of all time, and recently attended Game 1 of the Celtics playoff round vs. the 76ers. Obviously, the crowd went wild.