The sea around Phuket and Samui changes with the season: in the monsoon you need to watch the flags and your step, because waves and currents arrive and venomous jellyfish sometimes come close to shore. Below is what official sources say as of 19 October 2026, how to act after a sting and how to tell when it is better to stay out of the water. Currents are covered in detail in our answer on the red flag in Phuket; here we add to it and do not argue with it. The medical advice is general, and severe symptoms need a doctor.
Which sea hazards the sources name
| What | Where and when per the sources | Source |
|---|---|---|
| Box jellyfish | seen more often in the monsoon months; occasional reports: Phuket (Nam Bo Bay), Samui, Phangan, Krabi, Phetchaburi, Chumphon, Trat | TAT, 25 Aug 2025 |
| Fire jellyfish | October 2025: tourists at Patong and Kalim with a red rash and burning, no severe cases | DDC, 18 Oct 2025 |
| Portuguese man-of-war | 4 September 2024 at Patong; 29 May 2026 washed ashore at Karon and west Phuket, density 0.5 per 100 sq m, no injuries | DDC, DMCR |
| Others | TAT also lists the blue dragon (a sea slug), stonefish, weever fish and sea urchins | TAT, 25 Aug 2025 |
The UK Foreign Office writes that jellyfish can swim close to shore, particularly in the rainy season from May to October, and that stings can be fatal. The Government of Canada reports deaths from poisonous jellyfish off several islands and provinces. The sources we found have no statistics for individual beaches in Phuket and Samui, so the place names above are separate reports, not a risk map.
What to do after a jellyfish sting
The order, per the DDC and TAT:
- Get out of the water, call for help and call 1669. Keep the casualty as calm and still as possible and do not rub the sting
- Pour household vinegar (4-6%) continuously for at least 30 seconds. Vinegar does not relieve pain, but it stops the stinging cells from firing further
- Do not pour fresh water, white liquor or alcohol, and do not scrape or rub. If there is no vinegar, DMCR advises rinsing with seawater, not fresh water
- Remove tentacles still stuck to the skin with thick gloves or forceps, never bare hands (DMCR)
- If the pain does not pass, take the person to hospital. TAT asks for urgent help with severe pain, confusion, chest discomfort, difficulty breathing and changes in skin colour. If the person loses consciousness, start CPR until help arrives
For information about the illness the DDC line is 1422. TAT writes that first aid stations are available along many popular beaches, and DMCR asks beach operators to keep vinegar ready. Before swimming, ask a lifeguard or the management where the post is.
How to reduce the risk
- Do not swim after heavy rain, at night or outside the area marked by buoys (TAT)
- Look at the sand: if there are jellyfish remains on the shore, it is better not to enter the water (DDC)
- Covering clothing in the water, as the DDC advises
- Choose beaches with nets. TAT writes that protective nets are not installed everywhere, and DMCR advises swimming where they are
- Do not touch stranded jellyfish: they can sting on the beach too (TAT)
- Read the signs and listen to lifeguards: TAT writes that warning signs are posted where needed
Currents, flags and waves
The UK Foreign Office reports that beaches with lifeguards in Thailand run a flag system, and that people have drowned in strong rip currents. The Government of Canada asks visitors not to swim under a red flag. A red flag means one thing: do not enter the water.
Phuket. The west coast is open to waves in the southwest monsoon from May to October; more on that and on specific beaches in our answer on the red flag and in the guide to Phuket beaches without waves. A seasonal overview is in our guide to the low season.
Samui. According to the Thai Meteorological Department (TMD), the northeast monsoon blows from October to February. Red flags were raised on Samui’s beaches in February 2026, when a cold air mass over the Gulf of Thailand produced waves of about 1-2 metres at Chaweng and Lamai and swimming was banned along the whole coast (Khaosod English). In November 2024 the head of the Samui rescue centre said that in the monsoon season tourists are warned constantly at Chaweng and Lamai, where a red flag means no swimming. In the same month a large wave swept a tourist off the rocks below the Laad Koh viewpoint, after which the municipality closed access to the rocks: at the water’s edge on rocks it is dangerous in the monsoon even without swimming. More on the island in our Samui guide.
Boats, diving and insurance
The Government of Canada warns that passenger boat accidents have happened because of overloading and poor maintenance, and advises not to board a vessel that looks overloaded. Diving schools and rescue services may not meet international standards, so rent equipment from operators affiliated with major international training organizations. For adventure activities on the water Canada advises checking that your insurance covers evacuation, including by helicopter, and the activity itself. What to check in a policy is in our guide to insurance.
Who to call
| Situation | Number |
|---|---|
| Jellyfish sting, drowning, loss of consciousness | 1669, ambulance |
| Information about the illness | 1422, DDC |
| All numbers | emergency numbers in Thailand |
FAQ
Which jellyfish are dangerous in Phuket and Samui?
TAT names the box jellyfish as the main hazard and describes reports of it as occasional: Nam Bo Bay in Phuket, Samui, Phangan and other places. The UK Foreign Office warns that jellyfish stings can be fatal. In 2024 and 2026 the DDC and DMCR also wrote about the Portuguese man-of-war at Patong and Karon.
When is jellyfish season in Phuket and Samui?
The UK Foreign Office says jellyfish can swim close to shore, particularly in the rainy season from May to October. TAT says box jellyfish are seen more often in the monsoon months, when winds and currents bring them closer to shore. TAT gives no separate calendar for Samui.
What should I pour on a jellyfish sting?
The DDC advises household vinegar (4-6%) poured continuously for at least 30 seconds. Do not pour fresh water, white liquor or alcohol, and do not rub or scrape the sting. DMCR adds that if there is no vinegar you should rinse with seawater, not fresh water.
Can I swim when the red flag is up?
No. The UK Foreign Office and the Government of Canada ask visitors to follow warning signs and not to swim under a red flag. How to get out of a rip current is in our answer on the red flag in Phuket.
What if my skin burns and a rash appears after swimming and I did not see a jellyfish?
In October 2025 the DDC described exactly such cases at Patong and Kalim: red rash and burning, probably from a fire jellyfish. Follow the same steps: get out of the water, call for help, call 1669, do not rub, pour 4-6% vinegar for at least 30 seconds. If it does not get better, go to a hospital.
Sources
- TAT Newsroom. Jellyfish safety advice in Thailand: what tourists should know before swimming (25 Aug 2025)
- DDC. Jellyfish at Patong and Kalim, first aid, 1669 and 1422 (18 Oct 2025)
- DDC, Regional Office 11. Portuguese man-of-war at Patong, first aid (4 Sep 2024)
- DMCR (Department of Marine and Coastal Resources). Preventing jellyfish stings and first aid (25 Mar 2018)
- PRD (Government Public Relations Department). DMCR warning about the Portuguese man-of-war at Karon (29 May 2026)
- UK FCDO. Thailand travel advice: Safety and security (jellyfish, currents, flags)
- Government of Canada. Thailand travel advice (13 Oct 2026): currents, flags, boats, diving, insurance
- TMD (Thai Meteorological Department). Monsoons and seasons
- Khaosod English. Red flags raised as strong winds batter Samui (19 Feb 2026)
- Khaosod English. Russian tourist missing after being swept away by giant wave in Koh Samui (29 Nov 2024)
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