Best Time to Visit Koh Samui: Weather and Sea Conditions Month by Month

Samui is warm all year, so the best time to visit Koh Samui comes down to rain and the state of the sea. The island is on the Gulf of Thailand side of the country, and its weather runs on a different clock from Phuket and the Andaman coast. That catches a lot of visitors out.

In short, February to April is the most reliable stretch for sunshine and calm water. May to September is warmer, with short showers, and often a good time for boat trips. October to December is the main rainy season, and November is the wettest month. The numbers are below, along with what they mean month by month and how to plan a boat day around them.

Quick facts

Driest month	February (about 64 mm of rain, around 4 rainy days on average)
Wettest month	November (about 445 mm, around 16–17 rainy days)
Hottest months	April to August (average highs around 32–33°C)
Sunniest months	February and March (around 7.5 hours of sunshine a day)
Main rainy season	Mid-October to early December
Ang Thong Marine Park	Usually closed 1 November to 15 December
Climate data	World Meteorological Organization averages for Koh Samui, 1991–2020

Koh Samui weather by month at a glance

Averages for Koh Samui, 1991–2020. “Rainy days” are average days per month with measurable rain.

Don’t read too much into the rainfall totals. A high monthly figure can come from a few heavy downpours. March averages 116 mm, for example, but on under five rainy days. Outside the monsoon months, most showers on Samui are short and pass within the hour.

Why Samui’s seasons differ from Phuket’s

Thailand has two monsoons. The south-west monsoon (roughly mid-May to mid-October) brings the Andaman coast its long wet season, and Phuket’s rainiest months run from May to October. Koh Samui, on the other side of the peninsula, gets far less rain at this time. Long-term averages put it at roughly 100–135 mm a month, against roughly 210–360 mm in Phuket.

The north-east monsoon (roughly mid-October to mid-February) is the one that matters for Samui. As it arrives it brings heavy rain to the Gulf coast of southern Thailand, peaking around November. Across the whole year, Samui is a little drier than Phuket.

So a European summer holiday in July or August usually suits Samui better than Phuket. Between late October and early December it flips, and the Andaman side tends to be the drier choice.

Best time to visit Koh Samui, month by month

January

The tail end of the rainy season. Early January can still bring showers and lumpy seas, but conditions usually settle as the month goes on. Ang Thong has normally reopened by mid-December.

February

The driest month on average and one of the sunniest. Seas are generally calm, so it is one of the best months for any boat trip. It is also busy, so book tours a few days ahead.

March

Sunshine peaks and the heat starts to build. Rain usually comes as the odd heavy shower rather than grey days. The Thai National Parks guide to Ang Thong points to March and April for clearer water.

April

Hot, often calm and bright. Mid-April brings Songkran, the Thai New Year water festival, so expect water fights in busy areas. Bring a hat and plan swims for the morning and late afternoon if the heat bothers you.

May

The warmest average highs of the year. The south-west monsoon begins, but on Samui it mostly shows up as short showers rather than washed-out days. Generally a good, quieter month for boat trips.

June, July and August

Hot, with around ten rainy days a month, though most are brief. Sunshine hours dip as clouds build, but the sea on Samui’s south side is often calm enough for island-hopping. July and August are often busy with families on school holidays.

September

Much like August, and usually quieter. Now and then the Thai Meteorological Department issues strong-wind and high-wave warnings for parts of the Gulf, so watch the forecasts and your tour operator’s updates.

October

Rainfall more than doubles compared with September, and the north-east monsoon arrives in the second half of the month. Plan boat days with flexible dates, especially later in October.

November

The wettest month, with long rainy spells, strong winds and rough seas at times. Ang Thong Marine Park is usually closed from 1 November. Boat trips run only when conditions are safe, so many days are better spent on land.

December

The first half is often still wet, and the second half usually improves. Ang Thong normally reopens around mid-December. Christmas and New Year are among the busiest weeks of the year, so book early.

Planning boat trips around the weather

• Book your boat day early in your stay. If the weather forces a change, you still have days to reschedule.
• Watch the wind as well as the rain. A sunny day with strong wind can be rougher on the water than a cloudy, still one. Boats sail or stay in port because of waves and wind.
• Know the policy. On the Samui Journey catamaran, the team checks conditions every day. If it is not safe, we contact you in advance by WhatsApp, LINE or email and move you to another date free of charge. If the new date is also affected, you get a full refund. Full details are in our weather and refund policy.
• Pick the right trip for the season. Ang Thong means a longer open-water crossing and closes for six weeks every year. Trips to the islands off Samui’s south coast make shorter hops. Our Ang Thong or Pig Island comparison explains the trade-offs.
• Snorkellers: visibility is generally best in the calmer months. See our guide to the best snorkelling spots.

A plan for rough-sea days

When the sea is too lively for a boat, it is easy to swap in a land day.

• Samui Elephant Kingdom: about three hours at an ethical elephant sanctuary we support, with morning or afternoon hotel pickup. You learn what elephants eat, make “Powerball” elephant food and feed the elephants from the Skywalk. Lunch and soft drinks are included. Adults ฿3,000, children (5–11) ฿1,700, under-5s free.
• Explore the Real Samui 4×4 Island Safari: a full day in a 4×4 pickup, stopping at the Big Buddha Temple, a village experience in Chaweng, Namuang Waterfall, a rubber-tapping demonstration and Grandfather and Grandmother Rocks. Lunch is included, and the itinerary may change with the weather. Adults ฿1,950, children (5–11) ฿1,650, under-5s free.

Good to know

• Climate averages are not forecasts. Dry months can have stormy days and wet months can have sunny weeks. Check the forecast a few days out.
• Jellyfish: Thailand’s Department of Marine and Coastal Resources has warned that jellyfish risk around Koh Samui is higher from July to December. Many beaches have an orange pole with vinegar and first-aid instructions.
• Sun strength: even in the “cooler” months, highs sit around 29°C and the sun is strong on the water. Our boat day packing checklist covers sun protection.
• Ang Thong dates change slightly each year. The park usually closes from 1 November to 15 December, but the exact dates are announced annually and can shift with the weather.

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