The Portuguese men are back on the beach...and not the good kind.

Officials in Edgartown are warning beach goers of the Portuguese Man o' War jellyfish that have been spotted off Martha's Vineyard.

These large, blueish-purple jellyfish have a sail-like structure on them that allow them to float on the water's surface with their tentacles of up to 30 feet long dangling in the water below.

Many end up washed up on shore, where they can cause a seriously painful sting, whether they are alive or dead. In fact, they can still sting you weeks after washing up!

Of course, paying attention to where you swim is very important, but watching where you walk on the beach is also key. Once the Portuguese Man O' Wars wash up on shore they be hard to see when they are covered in sand.

Last summer, these crazy looking jellyfish were found as far inland as Westport, though so far this season only sightings on Martha's Vineyard have been reported.

If you're doing any late summer travel, be sure to keep an eye out as the sting from the Portuguese Man o' War can cause serious welts on your skin and are extremely painful.