DCR said Lion's Mane jellyfish were seen at Nahant Beach Reservation in Nahant, Wollaston Beach in Quincy, and Carson Beach in South Boston.
The agency also put up purple flags to warn swimmers about the jellyfish.
Lion's Mane jellyfish began arriving in large numbers in Massachusetts last year, puzzling local scientists. The Jellyfish can have a painful, but usually harmless, sting. They are usually known for their enormous size, up to six feet across and with trailing tentacles that can stretch to 100 feet.