The Hollywood star shared the news in an interview with David Letterman after the TV chat show host commented on how much weight Tom had lost.
"I went to the doctors and they said, 'You know those high blood sugar numbers you've been living with since you were 36, well you've graduated, you've got Type 2 Diabetes young man,'" said Tom.
"It's controllable," he added. "Something's going to kill us all, Dave."
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The Forrest Gump actor was told by doctors he would have to follow a stricter diet, and aim to weigh as much as he did when he was a teenager.
"My doctor said, 'If you can weigh as much as you weighed in high school you will essentially be completely healthy and will not have Type 2 diabetes,'" said Tom. "And I said, 'Well, I'm going to have Type 2 diabetes, because there is no way I can weigh as much as I did in high school."
It seems as though the Oscar-winning star has taken his doctor's advice, and was pictured looking noticeably slimmer while promoting his latest project, Captain Phillips. The movie is set to open the London Film Festival on 18 October.
Other celebrities who suffer from Type 2 diabetes, which sees the body unable to produce enough insulin to function properly, include Halle Berry, Patti Labelle and Sir Steve Redgrave.