Short answer
Koh Samui in November is warm but wet: it is the wettest month of the year, with about 445 mm of rain on roughly 16 days, according to the 1991 to 2020 normals. The Gulf of Thailand is also at its roughest, with waves of 2 to 4 metres at times in the Thai Meteorological Department's outlook. The Ang Thong marine park is closed from 1 November to 15 December 2026. Pig Island, Koh Tan and Koh Tao boat days still run when the sea allows, and our Pig Island day starts from $50 per person.
November is the wettest month on Koh Samui
The answer in numbers: Koh Samui receives 444.6 mm of rain in an average November, on about 16.5 days with at least 1 mm. These are the 1991 to 2020 normals for the Ko Samui weather station, published by the World Meteorological Organization from Thai data. No other month comes close. October brings 294.9 mm and December 263.1 mm, while February, the driest month, brings 64.3 mm.
November alone supplies more than a fifth of the island's yearly rain, which the same table puts at 1,994 mm. Samui's peak also comes later than on the Andaman coast: the Phuket station's normals give 336.0 mm in October but 178.2 mm in November. If you want the dry season on Samui, the Ko Samui table points to February and March, each with fewer than 5 rainy days. November is the other end of the scale.
How the weather feels: warm, humid and often grey
Rain does not mean cold. The same normals give an average November daytime high of 29.7 degrees and an average night low of 24.1 degrees, with relative humidity of 85.5 per cent, the highest of any month. In April and May the average high reaches 31.8 and 32.9 degrees, so November is a little cooler by day, but the air is damp and clothes dry slowly.
Averages hide the pattern of a wet month: sudden heavy downpours, then dry hours, then another band of rain. The Thai Meteorological Department's outlook for the 2024 to 2025 winter expected thunder rain over 60 to 80 per cent of the Gulf coast between late October and early January, with heavy rain in many areas and flash floods in some. Pack for water, and keep one indoor plan for each day.
Why the sea is rougher in November
The cause is the north-east monsoon. In its three-month outlook issued on 30 September 2024, the Thai Meteorological Department said the monsoon and a low-pressure trough over the Gulf of Thailand would keep southern Thailand wet in November, especially along the east coast, which is the Samui side. It also warned that tropical cyclones could pass near southern Thailand in October and November.
The Department's winter outlook, issued on 27 September 2024, gave sea figures: wind waves in the Gulf of Thailand at times 2 to 4 metres high between late October and early January, against 1 to 2 metres in the Andaman Sea. These are seasonal outlooks for one year, not forecasts for 2026, but the pattern repeats: November boat days on the Gulf depend on the day, and the crossing to Koh Tao is the one that feels it most.
Ang Thong is closed from 1 November to 15 December 2026
Ang Thong National Marine Park, the archipelago of 42 islands off Samui, closes every year for the monsoon. Asian Trails, reporting on 25 September 2026, gave this year's dates as 1 November to 15 December 2026, a closure of 45 days. The park reopens on 16 December. No day trip can land on the islands in between, whichever company sells it.
Our Ang Thong day follows the same dates: it runs before 1 November and again from 16 December. If Ang Thong is the point of your trip, either come in the first half of December and book the days after the 16th, or plan for January to April instead. The price and closure guide has the park fee, and Koh Samui in December covers the reopening.
What still runs in November: Pig Island, Koh Tan and Koh Tao
Two kinds of boat day carry on through the closure. The first is short and close to home: on our Pig Island and Koh Tan tour you snorkel at Koh Tan and spend free time on Pig Island, about 5 to 6 hours in total, from $50 per person. A fuller island day with a viewpoint, lunch and an elephant visit before the boat is from $65. Pickup covers Mae Nam, Bophut, Choeng Mon, Bang Rak, Chaweng and Lamai.
The second is the long crossing to Koh Tao and Koh Nang Yuan, from $55 per person. It is about 1 to 1.5 hours each way over open sea and, as the operator notes, it can be bumpy, mostly from late October to December. Entrance fees are paid in cash on the day: 250 baht per adult and 120 per child on that tour, while Nang Yuan's own website lists 250 baht per person. The fees and rules guide explains the island.
Who should come in November, and who should wait
November suits travellers who can live with a changeable day: you accept some rain, you treat boat days as flexible, and you do not need a particular island. It does not suit anyone whose trip hangs on Ang Thong, on calm, clear snorkelling water, or on a fixed single day for the boat. Our details also say that the Pig Island day is not recommended for travellers prone to seasickness.
For every boat day, the safety rule is the same: if the sea is unsafe, the operator moves the trip to another date or refunds it. That protects you, but it also means a one-day window can fail. Leave a spare day in your week, book the boat early in your stay, not on the last day, and compare the Gulf with the Andaman side in our guide to choosing a coast.
Loy Krathong and rainy-day plans in late November
One festival falls in the month. Loy Krathong takes place on the night of the full moon of the twelfth Thai lunar month, and Wikipedia's table of scheduled dates gives 24 November 2026 for this year. Ask your hotel about the local programme nearer the time, because events are set by each town.
On wet days the island still has options that do not depend on the sea. The Koh Samui things to do guide lists them, and the Big Buddha guide covers one of the easiest. If the whole week looks grey, remember that the Andaman coast is entering its drier months, and our Phuket in November guide shows what that side looks like.
Practical checklist
- Do not plan an Ang Thong day between 1 November and 15 December 2026: the park is closed.
- Book Pig Island or Koh Tao early in your stay and keep one spare day in case the boat is moved.
- Pack a light rain jacket, quick-dry clothes and a waterproof pouch for your phone.
- Take seasickness tablets for the Koh Tao crossing if you need them, and ask a pharmacist about the right ones.
- Keep cash for entrance fees on the day, such as Koh Nang Yuan's 250 baht per person.
- Check the Thai Meteorological Department forecast for the Gulf the evening before a boat day.
Questions and answers
Is November a bad time to visit Koh Samui?
It is the wettest month, with about 445 mm of rain on roughly 16 days in the 1991 to 2020 normals, and the Gulf sea is rough at times. It is not bad if you accept changeable weather and flexible boat days.
Is Ang Thong Marine Park open in November?
No. According to Asian Trails, reporting on 25 September 2026, the park is closed from 1 November to 15 December 2026. It reopens on 16 December, and no boat can land in between.
How hot is Koh Samui in November?
The average daytime high is 29.7 degrees and the average night low 24.1 degrees, with relative humidity of 85.5 per cent, according to the 1991 to 2020 normals. It feels warm and damp.
Are boat tours cancelled in November?
Short trips such as Pig Island and Koh Tan run in November. If the sea is unsafe, our operator moves the trip to another date or refunds it, so keep a spare day in your plans.
Is the Gulf of Thailand rough in November?
It can be. The Thai Meteorological Department's 2024 to 2025 winter outlook expected waves of 2 to 4 metres at times in the Gulf, against 1 to 2 metres in the Andaman Sea. That is an outlook for one year, not a 2026 forecast.
Is it better to be on the Andaman coast in November?
Often, for the sea. The same outlook expected thunder rain over 40 to 60 per cent of the Andaman side in November, against 60 to 80 per cent on the Gulf coast between late October and early January.