Ali, 72, was admitted to a hospital in an undisclosed location on Saturday morning (local time).
"The Ali family continues to request privacy and appreciates all of the prayers and well wishes," spokesman Bob Gunnell said. "No further details are being released."
A three-time world heavyweight champion and widely recognized as one of the best fighters ever, Ali, who suffers from Parkinson's disease, made a rare public appearance in September to attend a ceremony in his hometown of Louisville, Kentucky, for the Muhammad Ali Humanitarian Awards.
Ali, nicknamed "The Greatest," was diagnosed with Parkinson's about three years after he retired from boxing in 1981 with a 56-5 record.