Box jellyfish live in Thai waters in both the Gulf of Thailand and the Andaman Sea, and their stings can kill. The risk is highest in the rainy months and in shallow water after rain; if someone is stung, get them out of the water, call 1669, do not rub the skin, and pour vinegar on the sting for at least 30 seconds.
Most jellyfish you meet in Thailand cause a painful rash and nothing more. Box jellyfish are different: Thailand's Department of Disease Control (DDC) counted 48 serious injuries and deaths from box jellyfish between 1999 and 2021, and a 13-year-old boy died after being stung at a beach in Nakhon Si Thammarat on Saturday 1 August 2026. That is a small number next to road deaths, but it is enough to learn the first-aid steps before you swim.
Our article on dangerous animals in Thailand gives the overview of sea creatures, and the stonefish warning for Koh Lipe covers a hazard that needs the opposite treatment (hot water, not vinegar). This page is only about jellyfish: where, when, what to do and what not to do.
Box jellyfish: the essentials (checked October 2026)
- Where
- Eastern coast of the Gulf of Thailand and the Andaman Sea; often seen off Koh Samui, Koh Phangan and Koh Tao
- When
- Rainy season, especially in shallow coastal water after rain; July–October around Samui, Phangan and Tao
- Ambulance
- 1669
- First aid
- Out of the water, keep still, do not rub, household vinegar (4–6%) poured for at least 30 seconds
- Never use
- Fresh water, drinking water, alcohol or urine
- Prevention
- Long-sleeved protective swimwear, follow warning signs and lifeguards
Why box jellyfish are more dangerous than other jellyfish
Box jellyfish are named after their cube-shaped bell. They are nearly transparent and hard to see in the water. According to the DDC, the species with a single tentacle per corner can cause muscle pain and nausea, while the multi-tentacled species is the dangerous one. A heavy dose of venom can cause dizziness, cardiac arrest and respiratory failure within minutes.
A researcher at the Middle Gulf of Thailand Marine and Coastal Resources Research and Development Centre, a unit of the Department of Marine and Coastal Resources (DMCR), has said that the box jellyfish in Thai seas are as deadly as those in Australia. Their tentacles are long and carry many stinging cells. The DDC warns that a person who stays in the water after a sting can pass out and drown, which is why getting out of the sea is the first step of every official instruction.
Not every sting is a box jellyfish. In October 2025 several tourists at Patong and Kalim beaches in Phuket were stung, and the DDC said the likely culprit was a Chrysaora (sea nettle) jellyfish that causes a local rash. Patong's lifeguards treated them with vinegar from their first-aid points. You cannot tell the species apart in the moment, so treat every serious sting the same way.
Where in Thailand the risk is highest
Box jellyfish are found along the eastern coast of the Gulf of Thailand and in the Andaman Sea. The best-documented area is the trio of Gulf islands: the DMCR's research center in the middle Gulf has reported box jellyfish off Koh Samui, Koh Phangan and Koh Tao regularly from July to October. In 2019 officials put up warning signs at the beaches on all three islands, with bottles of vinegar next to the signs. We could not confirm that every post is still stocked in 2026, so carry your own small bottle if you swim there in those months.
The August 2026 death happened on the mainland Gulf coast, at Ban Fai Tha beach in Sichon district, Nakhon Si Thammarat. After it, the local national park raised its box jellyfish warning level and placed staff on the beaches. On the Andaman side, stings are reported from Phuket and the Krabi coast, though official sources do not single out one beach as a hotspot. Our guides to Koh Phangan's beaches and Phuket's beaches cover the individual beaches.
When: the rainy season and the hours after rain
The DDC says jellyfish are more common in the rainy season, especially in shallow coastal water after rainfall. The UK government's travel advice says jellyfish can swim close to shore, particularly in the rainy season from May to October. Note the timing on the Gulf islands: their heaviest rain usually comes from October to December, yet the box jellyfish season reported around Samui, Phangan and Tao runs from July to October. Do not assume calm, sunny Gulf weather in August means no jellyfish.
Two simple signals from the DDC: avoid swimming right after heavy rain, and stay out of the water when dead jellyfish are lying on the beach. Both mean there are probably more in the shallows.
First aid, step by step (DDC guidance)
These are the steps published by Thailand's Department of Disease Control in October 2025, repeated after the August 2026 death:
- Get out of the water and shout for help. Call the emergency medical number 1669. Do not stay in the sea, because the person can pass out and drown.
- Keep the person still and do not rub the area. Rubbing pushes more venom into the skin.
- Pour household vinegar (4–6% acidity) continuously over the sting for at least 30 seconds. Vinegar stops stinging cells that are still on the skin from firing.
- Do not use fresh water, drinking water, alcohol or urine. The DDC names all four as things to avoid.
- Go to hospital if symptoms continue or get worse. If the person stops breathing or has no pulse, start CPR and keep going until help arrives.
Do not touch any tentacles with bare hands, and keep watching the person's breathing until help arrives. Our page of emergency numbers lists 1669 and the other services if you need them.
Urine on a jellyfish sting is a myth that can make it worse. The DDC lists urine together with fresh water and alcohol as things not to use. Only vinegar, poured for at least 30 seconds, and a fast call to 1669.
Stinger suits, nets and other prevention
The DDC advises wearing protective swimwear that covers most of the body: long sleeves and long legs. A thin full-length rash suit or "stinger suit" protects the skin the tentacles would otherwise touch, and it also protects you from the sun. It is a sensible choice for children, who are less likely to notice warning signs.
- Swim where lifeguards are on duty and follow their instructions and the posted signs. Our guide to beach flags and rip currents explains the flag colors.
- Ask your hotel or a dive shop about recent sightings; the UK government suggests taking local advice from hotels and dive centers.
- Avoid swimming at dusk and at night in the risk months, when you cannot see anything in the water.
- Carry a small bottle of vinegar in your beach bag on the Gulf islands from July to October.
The DDC has urged coastal authorities to survey beaches, post signs, keep vinegar at risky beaches and consider jellyfish nets. Where a netted swimming area exists, swim inside it.
Snorkeling and diving trips are not exempt. A full-length rash suit costs little and is worth wearing on boat trips in the rainy months. If you are learning to dive, ask the school what it carries on board; our page on diving on Koh Tao has more on choosing a school.
Frequently asked questions
Are there box jellyfish in Thailand?
Yes. They are found along the eastern Gulf of Thailand coast and in the Andaman Sea, and are seen regularly off Koh Samui, Koh Phangan and Koh Tao from July to October. Thailand recorded 48 serious injuries and deaths from box jellyfish between 1999 and 2021.
What should I put on a jellyfish sting in Thailand?
Household vinegar with 4–6% acidity, poured continuously for at least 30 seconds, after getting out of the water and calling 1669. Do not rub the area and do not use fresh water, drinking water, alcohol or urine.
When is jellyfish season in Thailand?
Jellyfish are more common in the rainy season, roughly May to October, and in shallow water after rain. Around Koh Samui, Koh Phangan and Koh Tao, box jellyfish are reported mostly from July to October.
Is it safe to swim in Thailand because of jellyfish?
For most people on most days, yes. Serious stings are rare. Lower the risk by swimming where lifeguards work, wearing a long-sleeved swimsuit in the risk months, staying out after heavy rain and knowing the vinegar first aid.
Sources
- The Nation – Department of Disease Control: 4 dos and don'ts after contact with venomous jellyfish (19 October 2025) (checked October 2026)
- The Nation – Department of Disease Control: beware of box jellyfish (1 November 2022) (checked October 2026)
- The Nation – Boy aged 13 dies after box jellyfish exposure in Nakhon Si Thammarat (4 August 2026) (checked October 2026)
- The Nation – Tourists warned of box jellyfish at Surat Thani's popular islands (DMCR research center, 2019) (checked October 2026)
- UK FCDO – Thailand travel advice: Safety and security (checked October 2026)