Short answer
Koh Samui in December is better than November but not yet dry: the average is 263 mm of rain on about 12 days, down from 445 mm in November, according to the 1991 to 2020 normals. The Ang Thong marine park reopens on 16 December 2026 after its annual closure. The Thai Meteorological Department expects heavy rain on the Gulf coast until early January and waves of 2 to 4 metres at times. Our Koh Tao and Nang Yuan day starts from $55 per person.
December is wet, but clearly drier than November
The 1991 to 2020 normals for the Ko Samui weather station, published by the World Meteorological Organization from Thai data, give 263.1 mm of rain in December on about 11.6 days with at least 1 mm. In November the figures are 444.6 mm and 16.5 days, so December has roughly 40 per cent less rain. January drops again to 126.8 mm and February to 64.3 mm.
An average hides a lot. The same table lists the lowest and highest December totals of the 30 years as 27.3 mm and 816.2 mm. That is a drizzle in one year and a flood in another, so treat the average as a guide and not a promise. Plan December on Samui as a month that improves as it goes, not as a month that is dry.
The turning point falls in the middle of the month
The Thai Meteorological Department described the pattern in its three-month outlook issued on 30 September 2024. For southern Thailand, abundant rain in December mostly appears in the first half of the month, especially along the east coast, because the north-east monsoon still prevails over the Gulf of Thailand. The Department also noted that tropical cyclones could still move near the lower south in December.
Its winter outlook, issued on 27 September 2024, runs a little longer: heavy rain on the Gulf coast from Chumphon province downward until early January, then less rain from the middle of January as the monsoon weakens and the winds turn east or south-east. These are seasonal outlooks for one year, not 2026 forecasts, but they explain why the second half of December usually feels different from the first.
What it feels like: warm, with the coolest nights of the year
December is not cold on Samui. The normals give an average daytime high of 29.3 degrees and an average night low of 24.0 degrees, which is the lowest night average of the year. Relative humidity is 82.6 per cent, down from 85.5 in November. January has a slightly cooler daytime average of 29.0 degrees.
In practice that means light clothes, a rain jacket for the showers and a dry bag for your phone. The tables we checked do not give sea temperatures, so we do not quote a figure. If you plan to swim and snorkel in December, send us your dates on WhatsApp and we will tell you which days look best before you pay.
Ang Thong reopens on 16 December 2026
Ang Thong National Marine Park closes every year for the monsoon. Asian Trails reported on 25 September 2026 that this year's closure runs from 1 November to 15 December 2026, which makes 16 December the first day visitors can land again. Wikipedia describes the park as 42 islands in about 102 square kilometres.
On our Ang Thong day, from $50 per person, the speedboat carries up to 35 guests and the crossing takes about an hour. The park fee is paid on the day and is not in the tour price. The climb to the viewpoint is steep and takes about 15 minutes, and swimming in the Emerald Lake is not allowed. Read the price and closure guide for the full costs.
The sea in December and the Koh Tao crossing
The Thai Meteorological Department's winter outlook expected 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. A boat to Koh Tao crosses open sea for about 1 to 1.5 hours each way, so it is the trip that feels the Gulf most. Our operator says the crossing can be bumpy and is roughest from late October to December.
The rule on our trips is simple and fair: if the sea is unsafe, the operator moves the trip to another date or refunds it. That is why a spare day matters in December. If you are prone to seasickness, ask a pharmacist about tablets before you travel, and choose a day when the forecast looks calm rather than a day dictated by your flight.
Koh Tao and Nang Yuan: the trip that runs through the closure
While Ang Thong is closed, the Koh Tao and Koh Nang Yuan day is one of the longer boat days available from Samui, and it runs again in December. On our Koh Tao and Nang Yuan day, from $55 per person, you spend about 8.5 hours away, with pickup between about 7:30 and 8:30 am and a return to your hotel between about 4:00 and 5:00 pm.
Koh Nang Yuan is open to day visitors only from 10 am to 3 pm, so the boat leaves the island at about 3:00. Entrance fees are paid in cash on the day: 250 baht per adult and 120 per child on this tour. The bay and the order of the stops change with the sea. For the stop-by-stop route and times, see the travel guide and the tour schedule.
Choosing between Ang Thong and Koh Tao in December
Both are full speedboat days of 8 to 9 hours, with a steep viewpoint climb of 15 to 20 minutes, and neither suits small children or older guests who find the climb hard. Ang Thong has a crossing of about an hour and a protected park of 42 islands. Koh Tao has a longer crossing, two islands and a Thai lunch ashore. Their prices are from $50 and from $55 per person.
Choose Ang Thong if you want a national park, kayaking and a buffet lunch on the beach, and your dates fall on or after 16 December. Choose Koh Tao if your dates fall before the 16th, or if snorkelling at Koh Tao and Nang Yuan appeals more. Our Koh Tao price guide has the costs, and Ang Thong or Pig Island compares a shorter option.
Practical checklist
- Book an Ang Thong day for 16 December 2026 or later: the park is closed until 15 December.
- Keep one spare day in your week for any boat trip, because the Gulf can still be rough until early January.
- Pack a rain jacket, quick-dry clothes and a dry bag for your phone.
- Carry cash for Koh Nang Yuan's entrance fee and the Ang Thong park fee.
- Use reef-safe sunscreen and leave fins at home for Nang Yuan, where they are not allowed.
- Ask us about recent sea conditions on WhatsApp a day or two before your boat day.
Questions and answers
Is December a good time to visit Koh Samui?
It is better than November but still wet, with an average of 263 mm of rain on about 12 days in the 1991 to 2020 normals. The second half of the month is usually the better half.
When does Ang Thong Marine Park reopen in 2026?
On 16 December 2026, according to Asian Trails, which reported the closure from 1 November to 15 December on 25 September 2026. Check the park before you travel in case the dates change.
Is the sea calm in December?
Not reliably. The Thai Meteorological Department's 2024 to 2025 winter outlook expected Gulf waves of 2 to 4 metres at times until early January. That is a seasonal outlook, not a 2026 forecast.
Does it rain all day in December on Samui?
Averages say rain falls on about 12 days of the month, not on all of them. Showers usually come as separate downpours, so keep one indoor plan for each day.
What is the Koh Nang Yuan entrance fee?
On our Koh Tao tour it is paid in cash on the day: 250 baht per adult and 120 per child. According to our operator, Koh Nang Yuan lists 250 baht per person on its own website.
Is Koh Tao a good alternative if I cannot go to Ang Thong?
It is a full day with two islands and snorkelling, but the crossing is longer and rougher. The tour is not suitable for pregnant women or for guests with back problems or heart conditions.