Koh Samui in December: Weather, Sea Conditions, Christmas Crowds and What to Do

Koh Samui in December sits at the tail end of the island’s rainy season. The first half of the month is often still wet and windy, while the second half usually settles, just as the Christmas and New Year crowds arrive. It is warm the whole way through, with highs around 29°C, so the questions are about rain, the sea and prices, not heat.

We run boat days from Bang Kao Pier all through December, and we cancel some of them. This guide gives you the real numbers, what the sea does, which days are busiest and what to do on good and bad days, so you can plan a December trip without surprises.

Quick answer: December averages about 263 mm of rain on 11–12 days, much less than November but still wet. The north-east monsoon can make the sea rough, especially before mid-month, so keep boat dates flexible. Ang Thong Marine Park is closed until 15 December 2026. From about 20 December to New Year, the island is at its busiest and most expensive.

Quick facts

Average temperatures	High 29.3°C, low 24.0°C (WMO averages, 1991–2020)
Average rainfall	About 263 mm, on around 11–12 days with 1 mm or more
December 2025, measured	229.4 mm at the Ko Samui station, about 34 mm below normal (Thai Meteorological Department)
Sea	Rough spells under the north-east monsoon; calmer days in between, more often late in the month
Ang Thong Marine Park	Closed 1 November to 15 December 2026; due to reopen 16 December
Busiest period	Roughly 20 December to the first days of January
Public holidays	5 December, 10 December and 31 December

Koh Samui in December: the weather in numbers

These are the World Meteorological Organization’s 30-year averages for the Ko Samui weather station (WMO climate normals, 1991–2020), with November and January either side for context.

December gets a little over half of November’s rain and about twice January’s. On average, around 19 days in the month stay dry or almost dry. When it rains it can be heavy, and a spell of two or three grey days is normal.

Single years vary a lot. In December 2025 the Thai Meteorological Department’s monthly summary recorded 229.4 mm at Ko Samui, slightly below normal. Even so, heavy rain on 5 December caused flooding in parts of Surat Thani province, and the monsoon stayed moderate to rather strong over the Gulf for most of the month.

First half versus second half

The averages hide a trend inside the month. The north-east monsoon is still active in early December, and that is when most of the heavy rain and rough sea tends to come. By the second half, more days are bright and the sea calms more often, though a late surge of wind is always possible. If you can choose, aim for the second half, and plan sea days with a buffer either way.

Sea conditions and boat-trip cancellations

For a day on the water, wind and waves matter more than rain. A cloudy, still morning can be lovely at sea. A sunny day with a strong north-easterly can be rough.

A real example: on 16 December 2025 the Thai Meteorological Department’s daily forecast put waves in the lower Gulf at 2–3 metres, about 2 metres off Surat Thani, and told small boats in the lower Gulf to stay ashore. On days like that, no responsible operator sails. A few days later the same water can be flat.

• Decisions are made day by day. Operators check the marine forecast and sea state shortly before each departure.
• Small boats are hit first. Longtails and small speedboats feel the chop far more than bigger, steadier boats, and are often the first to stay in.
• Short routes help. The islands off Samui’s south coast, Koh Tan and Pig Island, are a short hop compared with longer crossings out to Ang Thong.
• Snorkelling visibility drops after a rough few days, as the water gets stirred up.

Our weather policy

If the sea is not safe on your date, we move your 1-Day Catamaran to a new date first. If bad weather stops the new date as well, you get a full refund. Separately, you can cancel free of charge up to 24 hours before pickup; later cancellations and no-shows are non-refundable. The boat also needs at least 40 guests to sail, and if it doesn’t reach that, you choose a new date or a full refund.

The practical advice is simple to follow: book your boat day early in your stay, so a postponement still leaves you days to try again.

Ang Thong in December

Mu Ko Ang Thong National Marine Park is closed from 1 November to 15 December 2026 under the Department of National Parks’ official closure list, and is due to reopen on 16 December. Early reopening days can still be cancelled if the sea is rough. If your dates fall before the 16th, or you want a shorter crossing, see what to do when Ang Thong is closed and our Ang Thong or Pig Island comparison.

Christmas and New Year: crowds and prices

The last ten days of December are among the busiest of the year on Samui. Flights, rooms and ferries fill up, and prices climb with them.

• Book flights and rooms early. For stays over 24–31 December, earlier is usually cheaper, and the better-value rooms go first.
• Read the small print. Many resorts set minimum stays over New Year, and several charge a compulsory gala dinner if you stay on 24, 25 or 31 December. In published offers for the season we have seen these surcharges range from about 3,500 baht to more than 10,000 baht per adult, on top of the room rate.
• Book restaurants for Christmas Eve and New Year’s Eve. Popular beach restaurants are often full, and some switch to set menus.
• Book tours a few days ahead. Boat trips fill up in the holiday week. Free cancellation up to 24 hours before pickup makes early booking low-risk.
• Allow extra time. Roads in Chaweng and around the airport are slower than usual, so leave earlier for flights.

Early December, before about the 20th, is noticeably quieter and better value, which is a fair trade for a bit more rain.

Events and holidays in December

These dates are fixed or confirmed. Anything organised by venues changes each year, so check closer to the time.

• 5 December: King Bhumibol Memorial Day, also National Father’s Day. A Thai public holiday; banks and government offices close, and some Thai families travel.
• 10 December: Constitution Day, another public holiday.
• 16 December: Ang Thong Marine Park is due to reopen.
• 24 December: full moon. Koh Phangan’s Full Moon Party is usually held around the full moon; check the organisers’ dates.
• 25 December: Christmas is not a Thai holiday, but most resorts and many restaurants put on dinners.
• 31 December: New Year’s Eve is a public holiday. Central Samui in Chaweng held a free countdown with concerts and midnight fireworks on 31 December 2025, beach bars along Chaweng and Lamai hold parties, and Haad Rin on Koh Phangan has its own big night. Ferries to Koh Phangan are busy, so book ahead.

What to pack for Koh Samui in December

• A light rain jacket or poncho. A jumper is rarely needed.
• A dry bag or waterproof phone pouch.
• Quick-dry clothes and sandals with grip for wet steps and temple floors.
• Sunscreen and a hat. The sun is strong between showers, more so on the water.
• Seasickness tablets if you are prone to it; take them before the boat, not on it. Our seasickness guide has more tips.
• A cover-up for temples: shoulders and knees covered.

The full list for a sea day is in our boat trip packing checklist.

The best things to do on good days

• Go to sea. When the forecast is calm, take the chance. Koh Tan for snorkelling and Pig Island are the shortest island trips from Samui. See Koh Samui boat trips compared.
• Ang Thong after 16 December, if you are happy with a longer crossing and the sea looks settled.
• Beaches on the sheltered sides. In the monsoon, Chaweng and Lamai often fly red flags. Choeng Mon, Bophut and the west coast are usually calmer. Our best beaches in Koh Samui guide explains why.
• The quiet south and west. Laem Sor Pagoda, seafood at Thong Krut and sunset at Taling Ngam make a good dry-day drive.

The best things to do on wet days

• Samui Elephant Kingdom: about three hours at an ethical elephant sanctuary that Samui Journey supports and books visits to, with morning or afternoon pickup, so you can choose the drier slot. Adults ฿3,000, children (5–11) ฿1,700, under 5s free.
• Explore the Real Samui 4×4 Island Safari: works on showery days, with the itinerary adjusted for weather. From ฿1,950 per adult, ฿1,650 per child (5–11), under 5s free.
• Waterfalls after the rain: Namuang runs at its fullest in December. Go once the rain stops and stay out of fast, muddy water; our Namuang Waterfall guide explains the risks.
• Indoors: a cooking class, a Thai massage, or Central Samui’s shops and cinema. Twelve more ideas are in our Koh Samui rainy day guide.

Is Koh Samui in December a good idea?

For most people, yes, with the right expectations. December is warm, greener than the dry season, and the second half often brings good beach and boat weather. Christmas on the island is relaxed, and New Year’s Eve is lively.

The downsides are real. Early December can still bring several wet days in a row and rough seas, Ang Thong stays shut until mid-month, and the holiday week is crowded and expensive. If calm seas are the whole point of your trip, February to April is more reliable; compare the months in our best time to visit Koh Samui guide, or read about Koh Samui in November if you are deciding between the two.

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