The 52-year-old heir to the throne briefly lost consciousness after the accident, which happened as he was taking part in a charity match in Cirencester, Gloucestershire, in western England.
"He was unconscious for a very brief time, was taken to the local hospital and is now being transferred to Cheltenham General Hospital where he will stay for a precautionary overnight stay," spokesman Patrick Harrison said.
Earlier, a spokeswoman said the prince did not break any bones in the fall.
"Prince Charles had been playing quite well and had just scored a goal when it happened," said Lucy Darch, 25, a spectator at the match.
"The teams swap ends whenever a goal is scored so they had all just charged down to the other end of the polo field. He went to take a forehand on the right side but as he swept his arm forward, he went backward off his horse," Darch said.
A keen horseman and expert polo player, the 52-year-old prince has had his share of equestrian injuries, including a cut on his face that required 12 stitches.
In June 1990 he fell during a polo match, was kicked by a horse and suffered two fractures in his right arm. The break did not heal properly and the arm was reset three months later.
Three years later, he was hurt again during a game of polo when he aggravated an old back injury.
In January 1998 he broke a rib after falling from his horse while fox hunting, and in January of this year, he fractured a bone in his shoulder when he fell off a horse during a hunt.
The prince also suffers from a degenerative disc at the base of his spine, which has caused some pain over the years.
On Wednesday the Queen Mother, who will be 101 Saturday, was taken to the hospital suffering from anemia.
The mother to reigning monarch Queen Elizabeth II was discharged Thursday after undergoing a blood transfusion.
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