Koh Samui in December 2026: Rain, Sea Conditions and Where to Find Sun

December on Koh Samui splits neatly in half. The first ten days still carry the tail of the northeast monsoon: 30-40% chance of rain, 5-7 hours of sun a day, and seas ranging from moderate to calm. From 11 to 20 December the rain chance drops to 20-30%, sun rises to 7-8 hours, and the sea settles. Water sits around 28.5°C, with daytime highs of 29°C and nights near 24°C.

For travelers, and for anyone weighing a villa or hotel stay on the island, the date window matters far more than the monthly average.

Weather by Ten-Day Period

Monthly rainfall figures for December vary widely between sources: some report 100-150 mm (after November's 250-350 mm), others 163 mm, others 263-265 mm. Samui Journey cites roughly 263 mm over 11-12 rainy days. None of these are wrong. Sources use different weather stations and different averaging years, and the island's west and east coasts experience their own weather.

So plan by ten-day period and rainy-day count rather than by monthly totals. Sources agree on this much: it rains on about 11-12 days out of 31, and around 19 days are dry or nearly dry. Showers tend to be short and heavy rather than all-day.

• 1-10 December: 5-7 hours of sun, 30-40% rain chance, moderate sea that can turn choppy in places.
• 11-20 December: 7-8 hours of sun, 20-30% rain chance, calm sea.
• 21-31 December: the most sun and the lowest rain chance of the month, calm sea.

In the first week, two or three grey days in a row are entirely possible. That is normal for December, not a malfunction. If you only have five days and they fall on 2-6 December, keep at least one spare day in the plan.

Which Coast Has Calmer Water

The north shore, Maenam and Bophut, is churned by residual northeast wind early in the month: murky water, ripples, and sometimes an unpleasant shore break. The east coast, Chaweng and Lamai, flattens out faster and is usually smooth by mid-month.

That said, December wave height on this coast can reach 0.64 m, among the roughest readings of the year for the Gulf. Adults can swim without trouble, but small boats and children on inflatable rings will find it harder going.

The simplest approach: check the flags and the water in the morning, and if the north looks murky, head east. The island is small, and Bophut to Lamai is about 30 minutes by road.

A word on tides too. On shallow Samui beaches the water can retreat so far that you walk a long way to swim. It is not tied to December specifically, but it frustrates anyone who arrived in high season expecting a postcard.

Ang Thong Marine Park Reopens 15 December

Ang Thong National Marine Park is closed until 15 December. If the trip is the centerpiece of your holiday, shift your dates: you will not get in during the first ten days.

Other boat excursions mostly run in early December, and from mid-month everything operates. Snorkeling stops being a lottery on visibility. Still, remember the swell: the Gulf is rougher in December than the sunny sky suggests, and small boats are cancelled more often in the first ten days.

Book morning departures. The wind usually freshens by lunchtime, and the return trip is harder on anyone prone to seasickness.

When Hotel Prices Jump

Hotel and villa prices climb gradually from the start of the month but stay reasonable until about 20 December. After that the Christmas and New Year peak begins, and the same room costs a very different amount. One Russian-language market guide describes high-season rates as roughly double.

If your dates are flexible, 15-20 December is the sweet spot: Ang Thong is open, sun runs 7-8 hours, the sea is calm, and the New Year premium has not kicked in. If you want a lively beach New Year or are tied to school holidays, this advice is not for you. Book early, as good late-December options are taken months ahead.

Chasing the Sun When Samui Turns Grey

The key December insight for Thailand is that the Gulf and the Andaman Sea run on opposite seasons. While Samui finishes off the monsoon tail, Phuket, Krabi and Phang Nga are in their dry season. If the first ten days wash you out, flying to the Andaman side changes the weather far more than moving to the other side of Samui.

If leaving is not an option, you still have choices:

• waterfalls and viewpoints in the south and center, which actually look best when water levels are high;
• Nathon town and the local markets, where rain matters least;
• indoor spas and massage, which cost the same on a grey day as a sunny one;
• the east coast as a fallback beach on any day in the first ten days.

Avoid non-refundable boat tours for 2-8 December. That is where the money most often evaporates.

FAQ

Can you swim on Koh Samui in December?

Yes. Water temperature stays around 28.5°C all month. The issue is not warmth but waves and murky water on the north shore during the first ten days.

When is Koh Samui driest in December?

The second half of the month. From 11 December the rain chance falls to 20-30%, and by the end of the month the island is entering its dry season.

Is Ang Thong Marine Park open in early December?

No, the park is closed until 15 December. Boat tours there only start after it reopens.

How many days does it rain on Samui in December?

About 11-12 days of the month, with the other 19 dry or nearly dry. Rain tends to be short and heavy.

Is it worth visiting Samui on 1-10 December for lower prices?

If you mainly want a beach and can accept two or three grey days, yes: prices are not yet at their peak. If you want boat trips and snorkeling, move to the second ten-day period.

Where should I go if Samui is rainy?

The Andaman side, meaning Phuket, Krabi and Phang Nga, is in its dry season in December and offers a very different forecast from the Gulf.

What temperatures should I expect?

Daytime highs around 29°C and nights around 24°C, with Samui Journey citing averages of roughly 29.3°C and 24.0°C.

Should I watch wind or rain in the forecast?

Wind. In December the wind direction decides which coast you swim on, while the rain usually passes within half an hour.

Source: Samui Journey

Ready to invest in Thailand? Our experts will help you find the perfect property.