Zookeepers found George, 28, unable to stand Monday and euthanized him early Tuesday, The Denver Post reported Wednesday.
He had been suffering from diverticulitis, chronic diarrhea and weight loss and a preliminary necropsy showed the animal had a ruptured spleen. George was the ninth-oldest black rhinoceros in a zoo in the world.
George came to the Denver Zoo from the Cincinnati Zoo in 1984 when he was 1 year old. The rhino fathered a calf in 1992 named Tony.
The Denver Zoo is home to two other black rhinos: Shy Anne, a female, and Mshindi, a male.