The National Park Service said Friday that the visitor, who was not named, was bitten Thursday at Delaware Water Gap National Recreation Area.
Superintendent John Donahue said the person was swimming in the Delaware River just south of the park's Eshback Access near Bushkill at the time.
He said the animal was later tested for rabies and the test came back positive.
Donahue said he did not know whether other park animals had rabies, which is transmitted by the bites of infected animals.
Anyone who sees sick or dead wildlife there is being warned not to approach or touch the animals but to call the park's emergency center.