Jellyfish stings are the most common cause of marine envenomation in humans. Various species of box jellyfish have been identified around Penang Island, Malaysia, and these include multitentacled and four-tentacled box jellyfish (class Cubozoa). The typical syndrome following envenomation from these jellyfish has been poorly documented, posing a greater challenge when managing an unidentified jellyfish sting from Penang Island. We report a case of a 32-y-old man from Penang Island who was stung by an unidentified jellyfish while walking into the sea. The patient reported that he felt an immediate and severe electric current‒like pain over both thighs, left flank, and left forearm, followed by chest discomfort and breathlessness. Vinegar was applied over the affected areas, and he was rushed to a hospital, where he was treated with analgesia, steroids, and antihistamine. He refused hospitalization and was discharged against medical advice. He then presented to a noncoastal hospital 377 km away in Kuala Lumpur on the following day with severe pain over the affected sites as well as chest discomfort, shortness of breath, and abdominal cramps. The electrocardiograph demonstrated features of Wolff-Parkinson-White. Serial blood test results showed elevated creatine kinase but normal troponin I levels. The patient was managed symptomatically over a period of 4 d and was discharged with cardiology follow-up. Appropriate health-seeking behavior needs to be emphasized. This case report provides an opportunity to document the signs and symptoms of envenomation from possibly an undescribed jellyfish species near the coastal waters of Penang Island.