Koh Samui Weather by Month: When It Rains and When to Stay

Averages 2015-2025 · Source: Open-Meteo, CHIRPS · How we measure

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A route for your months with a leave-by date for each stop, a backup, and the seasonal traps on your path. Drafted from 11 years of weather data, every number checked against it, in your inbox within 48 hours. Not included: visas, insurance or bookings.

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What is the weather in Koh Samui, in one minute?

Warm all year, with one wet stretch. Daytime highs sit between 28°C and 31°C in every month and nights between 25°C and 27°C, so temperature is never the question. Rain is. The Gulf of Thailand gets its heavy rain late in the year: 260 mm in October, 457 mm in November and 318 mm in December. The dry window is February to April, with March the driest at 47 mm.

Read the chart above in two passes. The temperature band is nearly flat, rising a little to May and June. The rain line has one tall hump at the end of the year and a smaller one in May.

How does each month look?

The table puts every month side by side: highs, lows, rain days and rainfall.

A rain day here is a day with at least four millimetres, the tropical cut-off, so a drizzly day does not count. Cloud cover tells a second story our table leaves out: it is about half the sky in February but more than four fifths from May to November, so the wet months are also the grey ones.

When is the dry season in Koh Samui?

February to April. February has 79 mm, March 47 mm and April 90 mm, with cloud covering roughly half to two thirds of the sky. Highs climb from 29°C in February to 31°C in April. March is the best month of the year for rain: the least of it (47 mm), only about two rain days, and a comfortable 30°C high.

January is the shoulder. It is the coolest month alongside December (28°C), but it still carries 152 mm over about five rain days. If you can only pick one month of the dry window, pick March; if you want to start the Thai winter early, February is the safer start than January.

When is the rainy season in Koh Samui?

October to December, and November is the worst of it. The island gets 260 mm in October, 457 mm in November and 318 mm in December. Humidity climbs to 84% in October and November, up from 77% in the dry months. Beaches get rough, some boats and ferries cancel, and low roads can flood.

This surprises people who know Phuket. The Andaman coast is wet from May to October and clears in December; Samui is the reverse, wettest just as Phuket dries out. If you are moving between the two coasts, our guide to the three coasts of Thailand lines them up, and Where to go in November gives the places that are dry when Samui is not.

What about May to September?

Warm, grey and moderate. May brings 136 mm and a short bump in rain, then June to August fall to 86 mm, 106 mm and 75 mm. These are not dry months: cloud covers close to nine tenths of the sky, so you get hot, humid days broken by showers, not clear blue ones. Highs of 31°C in May and June are the hottest of the year.

It suits people who care more about cost and quiet than a perfect beach, and who work indoors. Do not mistake it for a second dry season; the rain is lighter than at the end of the year, but the sun is not out much. For the island pair, see Koh Samui and Koh Phangan: best time.

How does Samui compare with Hua Hin and Chiang Mai?

Both are much drier than Samui in November and December, which makes them the usual escape.

Hua Hin is on the same Gulf but far drier in November and December. Chiang Mai is inland and in its cool, dry season, with cooler nights. Neither has Samui's beach. For a full month-by-month on the northern option see Chiang Mai weather by month.

What would we do?

Match the stay to the rain. Come to Samui in February, March or April for a base with sun and warm water. In November and December, plan to be elsewhere: Hua Hin, Chiang Mai and Bangkok are all much drier then, and our Thailand in December page ranks them. October is wet almost everywhere in Thailand (Bangkok 315 mm, Hua Hin 276 mm), so of the three only Chiang Mai, at 161 mm, is drier than Samui that month. If you want Samui in the wet spell anyway, December is the least bad of the three months: more rain in total than October (318 mm against 260 mm), but it falls on about 10 days instead of 18, under less cloud. January is the recovery month. This is a weather view only: it does not cover flights, ferries, cost of living or internet speed, so check those before you book.