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, spokesperson 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 boxer's condition and said Ali's family was asking for privacy.
The boxing great, who suffers from Parkinson's 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 Parkinson's about three years after he retired from boxing in 1981.
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"Boxing Great Muhammad Ali", as opposed to the great pastry chef Muhammad Ali? Thanks, we all know who he is and why he was probably the most famous person in the world.
I wish speedy recovery, happiness, health and a long life for the charismatic superstar, a torchbearer, a living legend Muhammad Ali.
A great man and human, who has stood for his beliefs in the face of adversity, I highly admire him and pray for his speedy recovery.
My respects to Muhammed Ali for historic protest in Rome Olympics 1960-Black salute, Th progress we enjoy in western world is due to this Sub continent persons should thank Muhammed Ali for his bravery. With our discriminatory attitudes and terrorist tag we should be sent back from Britain,. USA, Canada, Norway, Sweeden. Japan, Milan Italy, Greece.