The closings Wednesday affect beaches from the Orleans-Chatham town line south along Nauset Beach to Monomy. They are in effect until further notice.
Other public beaches remain open in the town.
Swimmers are warned to keep aware of their surroundings, and stay at least 300 feet from seals.
Sightings of sharks, including great whites, have increased off Cape Cod in recent years along with an increase in the population of seals, which sharks feed on.
In July, a man was bitten on his legs by a great white shark while swimming off Ballston Beach in Truro, north of Chatham.