The animal made its way to the lake in the Fen Drayton Lakes Reserve run by the Royal Society for the Protection of Birds, the Cambridge News reported. The wildlife preserve is near St. Ives in Cambridgeshire.
A 29-year-old RSPB member filmed the seal.
"At first sight in the water I thought it was a dog in trouble but as it came up for air I could see it was a seal," the member said. "It was swimming up a ditch that links to the Great Ouse. In the video it is swimming from the ditch and up into the flooded field."
Graham Elliott, area manager for the RSPB, was excited by the sighting.
"It would be tremendous if one is there. It would be extremely unusual and it would become an attraction to visitors," he said.
Elliott said seals sometimes swim up the Great Ouse River but have never been seen in the preserve.
The preserve is in a low-lying area known as the Fenland. Once a stretch of marshes, it has been drained over the centuries, leaving isolated marshes or fens surrounded by agricultural land and connected by a network of rivers, canals and drainage ditches.