Koh Samui and the adjacent islands in the Gulf of Thailand have different high seasons than the rest of Thailand.

Usually you are used to going to Thailand from about November to March, when it rains the least. However, Koh Samui has a good climate from January to August.

• 👉 Seasons and weather throughout Thailand

Buy flights in high season in advance - skyscanner.com

What is the climate like on Koh Samui?

Koh Samui has a tropical climate - that is:

• High temperatures all year round
• high humidity
• alternating dry and rainy seasons

Temperatures stay between 28-32°C during the day and around 23°C at night for most of the year.

In other words: there is no winter here.

What changes more significantly, however, is:

• the amount of rain
• the state of the sea
• cloud cover

And that's what has the biggest impact on your holiday.

What is the weather like on Koh Samui during the year?

Unlike other parts of Thailand, Samui has a slightly delayed season.

To put it simply:

• January - August → mostly stable weather
• September → transition
• October - November → main rainy season
• December → return to a better season

👉 This is especially important if you're comparing Samui to Phuket or Krabi - the season works differently there.

Find out hotel prices in high season

Weather on Koh Samui month by month (reality)

Reality as we have personally experienced it on several trips to Koh Samui.

January - April

While the first half of January can still see overcast skies with occasional showers, the weather starts to improve significantly from mid-January onwards when the prevailing:

• light showers mainly in the evening or at night
• mostly partly cloudy (rarely clear skies)
• good and clear sea (unless there is a storm elsewhere in the Gulf of Thailand)
  • but in January there is often still a lot of algae in the sea after the autumn rainy season
• normally 26 °C to 29 °C

From the second half of January to the end of March are usually the best conditions for a trip to Koh Samui.

May - June

• Hot (most of the year) but relatively low humidity compared to other months
• Mostly still good weather, partly cloudy
• occasional showers
• sea conditions are good but algae clings to the bays (especially north Chaweng)
• normally 29 °C to 33 °C

💡 Also spring and early summer is a great time to go to Koh Samui.

July - August

• Still good weather with occasional showers, mainly after midday or at night
• partly cloudy, can be cloudy in the afternoon
• the sea is still quite clean, but there may be algae in the quieter bays
• normally 28 °C to 32 °C

💡 Koh Samui is the only Thai area where it is high season even during July and August. It generally has much better weather than say Phuket or Krabi.

Book a hotel in high season well in advance

September

• Transition period
• More volatile, heavy showers are more frequent, especially in the second half of the month
• but the sea is still quite clear

💡It can be great, but it can also be worse. But it is true that Koh Samui in September is the safest choice of all Thailand.

October - December

• Most rain, torrential downpours can occur every day
  • usually rains for 60-90 minutes in the afternoon or evening
  • but there can be cloudy skies for several days at a time
• worse sea, muddy and the shores are full of algae
• normally 27°C to 31°C

💡 The worst time of year for a beach holiday. At this time of the year you'd better go to Phuket, Koh Lanta or Krabi.

How much does it rain on Koh Samui?

This is important to understand correctly.

Rain in the tropics does not mean all day long.

Often:

• Short, intense showers
• rain in the evening or at night

BUT in October, November, December and early January:

• it can rain for long periods of time
• can be cloudy for long periods
• the sea tends to be worse

What is the sea like by season?

The sea changes more than one would expect.

• Best: February - August
• Worse: October - December

Differences:

• water clarity
• Waves
• colour
• a lot of algae on the shore

Water temperature in the sea on Koh Samui

The sea on Koh Samui is practically the same temperature all year round.

It varies between 27°C and 29°C everywhere on the island.