The Best Time to Visit Koh Samui, Month by Month, From a Crew on the Water

Why Koh Samui’s best months are the opposite of Phuket’s

Phuket and the Andaman coast take the south-west monsoon, roughly May to October, while Samui sits in the Gulf of Thailand, largely sheltered from it, and gets most of its rain from the north-east monsoon late in the year. So July on Samui is usually a good month while the Andaman side is at its roughest, and November is the reverse.

Koh Samui weather by month: the table

This is how the year looks from our deck. Weather varies from one year to the next, so read it as a pattern rather than a promise.

Late December to April: the best time to visit Koh Samui for calm seas

Late December to April is the best time to visit Koh Samui if the sea is a priority. Swell is low and the water is usually at its clearest for snorkelling. February and March are usually the driest months, and April is reliably calm, if very hot.

It is also peak season, so the island is full and the good departures go first. Our shared trips cap at 15 guests and website bookings close 24 hours before departure, so in January and April it is worth booking a few days ahead rather than the night before. In April, Songkran turns much of Thailand into a water fight for a few days in the middle of the month; expect busy roads and a cheerful soaking if you go out.

This is also the kindest window for the long crossings to Ang Thong National Marine Park, which is closed to visitors every year from 1 November to 15 December. We do not go there, but if it is on your list, Ang Thong by big boat with kayaking is the steadier way across.

June to August: the months people underestimate

June to August is a good time to visit Koh Samui, and it is the period most often written off by people who assume the whole of Thailand is wet in summer. Showers come, but they tend to be short, and the sea on Samui’s north and east coasts is usually workable. A breeze picks up on some afternoons, which is welcome in the heat.

July and August are busy with European school holidays, so families should book boat days early. On our half-day trip, children aged 4 to 11 get a supervised snorkelling session with the crew in shallow water at the snorkel stop.

Which month to avoid on Koh Samui?

If you can choose, avoid November, the middle of the north-east monsoon that runs from about mid-October to mid-December. Long-term averages put its rainfall at many times February’s, and it is the windiest stretch of the year on the water, so boat trips are most likely to be moved. Rain tends to come in bursts with sunny spells between, and the island is green, quieter and cheaper; Loy Krathong, the festival of floating offerings, usually falls on a November full moon. Our guide to the rainy season on Koh Samui covers how to plan around it.

What the north-east monsoon means on a boat

During the monsoon the wind comes from the north-east. The east coast takes the swell first. The north coast is sometimes easier, but a strong north-easterly makes it rough too; the crew decide on the day.

That matters for which trips run as planned. Our half-day trip runs to Silver Beach or Coral Cove on the east coast, so it is the one most likely to be rerouted when a swell is running; the crew can sometimes use the neighbouring coves, and they adjust the route for the sea state.

When it is not safe or not pleasant, we do not go. The crew cancel for weather or safety, and you choose a free reschedule or a full refund. A slow, heavy boat handles a lumpy sea far more kindly than a speedboat does, but there is a point at which nobody should be out there, and the crew make that call.

September and October sit either side of this. September brings more frequent showers but is mostly workable. The first half of October is usually the better half; the second half is when the monsoon tends to arrive.

How to plan boat days around Koh Samui’s weather

The single most useful thing you can do is put your boat day early in your stay. If the weather moves it, you still have days left to reschedule. This matters most from mid-October to mid-December, but it is good practice in any month.

• Book the boat for your first few days, not your last. A cancelled trip on your final morning is a lost trip.
• Favour mornings when the forecast is mixed. On many days the sea is flattest early, before the afternoon breeze builds; our half-day boards at 09:00.
• Watch WhatsApp the day before. We confirm your pickup time and boarding point, Petcharat Marina in Bangrak or Hansar Samui Resort in Bophut, and that is where we tell you if the plan changes.
• Pack for both. Sun cream and a hat in the dry months, when the reflection off the water is fierce; a light layer and a dry bag for your phone in the wet ones. The cabin and stern on our boat are shaded.
• Read the weather policy before you pay anyone. Ours is simple: if the crew cancel for weather or safety, you choose a free reschedule or a full refund.

For a group, a private charter gives you the most room to work around the weather: you can request a start time between 07:00 and 16:00, and the crew plan the route with you within what the sea allows. Weekends are kept for charters; weekday charters depend on availability.