A shark of “significant size” was spotted off Hempstead, LI, on Monday — prompting local beach closures, officials said.

The town tweeted out an alert in the early afternoon saying, “A significant size shark was spotted offshore of Lido West Beach.

“The shark was spotted by a Town of Hempstead lifeguard on a surfboard. Due to the shark sighting, the lifeguards have red-flagged the strip of water as per the NYS shark protocols.

“Town of Hempstead Bay Constables have been dispatched to assist with the search.”

The “Shark Spotting Alert” said Civic, Lido and Lido West beaches were closed as a precaution, as were Town Park Point Lookout and Town Park at Sands.

Long Beach also was closed to swimmers because of the sighting.

“The Town of Hempstead has alerted us to a shark sighting in nearby waters,” the city of Long Beach tweeted just after 11 a.m.

“As a precaution, all City of Long Beach beaches are closed to swimming until further notice.”