The jellyfish, known as the mauve stinger or "purple meany," has been moving along the Jersey Shore and stinging multiple swimmers.
Among those affected were a group of children who were stung over the weekend while swimming off Manasquan.
George Slager suffered a serious injury and required immediate attention. His mother took photographs of welt marks and abrasions shortly after he was stung by a jellyfish. Slager said the pain came on quickly.
While Slager's injuries are healing, his brother was also stung by a mauve stinger. He has markings around his ankle that resemble a poison ivy rash.
Their mother later posted photos on social media and discovered that many other swimmers had also been stung over the weekend.
Officials said the mauve stinger is native to local waters but typically remains well offshore. At times, however, the jellyfish move closer to shore, increasing the likelihood of encounters with swimmers.
Authorities are urging beachgoers to remain aware of conditions in the water and take precautions to protect themselves and their families.