The 72-year-old three-time world heavyweight champion was admitted on Saturday, though his spokesman indicated Ali was expected to make a quick recovery.
“Because the pneumonia was caught early, his prognosis is good with a short hospital stay expected,” said Bob Gunnell, who did not reveal further details.
Ali’s wellbeing has been the subject of speculation in recent months after his brother Rahman said in October that the iconic fighter could barely speak or leave the house due to the effects of Parkinson’s disease.
It prompted his doctor, Abraham Lieberman, to last month to deny Ali was not gravely ill, saying: “I don’t see anything immediately that leads me to think that he is going to die in six months or a year.”
Ali was diagnosed with Parkinson’s shortly after calling time on one of sport’s most celebrated careers in 1981.