He suffered a "massive heart attack" last night and died this morning. His family released a statement -
It is with profound sadness that the family of George M. Steinbrenner III announces his passing. He passed away this morning in Tampa, Fla., at age 80. He was an incredible and charitable man. First and foremost he was devoted to his entire family - his beloved wife, Joan; his sisters, Susan Norpell and Judy Kamm, his children, Hank, Jennifer Jessica and Hal; and all of his grandchildren. He was a visionary and a giant in the world of sports. He took a great but struggling franchise and turned it into a champion again.
The funeral will be private, "but there will be an additional public service with details to be announced at a later date."