Ali, 72, was admitted to a hospital in an undisclosed location Saturday morning and is being treated by a team of doctors and remains in stable condition, spokesman Bob Gunnell said.
"Because the pneumonia was caught early, his prognosis is good with a short hospital stay expected," Gunnell said in a statement.
He declined to give any further details of the boxers condition and said Alis family was asking for privacy.
The boxing great, who suffers from Parkinsons disease, made a public appearance in September to attend a ceremony in Louisville, Kentucky, for the Muhammad Ali Humanitarian Awards.
A three-time world heavyweight champion, Ali was diagnosed with Parkinsons about three years after he retired from boxing in 1981.(*)