Weather in Koh Samui in May

How May compares

May is the 1st warmest month of the year in Koh Samui and ranks 16th of 134 destinations we compare for May - see where else is hot in May. Within Thailand it is 3rd of 6 (Thailand in May). Against London's average high of 16.6°C in May, Koh Samui is 13.5°C warmer by day and 19.0°C warmer at night.

Daytime and night-time in May

The typical May day in Koh Samui peaks at 30.1°C, but that is an average: the warmest 10% of days reach 31.4°C or more. About 100% of days top 25°C and 1 in 2 pass 30°C. Nights average 27.3°C; one night in ten drops to 26.1°C or lower, so pack a layer for evenings. Between 1991 and 2020 the hottest May day here reached 33.8°C (2020-05-18) and the coldest night 24.4°C (2007-05-04).

Rain in May

Wet (more than 8 rainy days): about 22 days with 1 mm or more, 119 mm in total. Year to year it varies a lot - the wettest May was 1991 with 229 mm, the driest 1997 with 45 mm. Humidity averages 78%, which makes the heat feel heavier than the thermometer suggests.

Sea temperature in May

The sea averages 30.5°C - warm enough to swim (22°c+). Depending on the year it ranges from 29.7°C to 31.1°C. This is the warmest the sea gets here all year.

Sunshine and daylight

Around 2.7 hours of sunshine a day (estimated from cloud cover), 12.4 hours of daylight: the sun rises about 06:00 and sets about 18:33 local time in mid-May. Average cloud cover is 78%.

Is May a good time to visit Koh Samui?

It is a reasonable choice: hot days but expect showers. The best months for Koh Samui on our scoring are February, March, January; months averaging 25°C or more: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec.

Hot days with little overnight relief

May is Koh Samui's warmest month, so choose it because you enjoy heat, not because you expect gentle warmth. Average daytime temperatures are 13.5°C higher than London's in May, and humidity of 78% adds to the sticky feel. Loose, lightweight clothing makes sense, with strenuous outdoor plans kept away from midday where possible. Nights average 27.3°C, so air conditioning is a sensible accommodation priority if you struggle to sleep in warmth. Evenings may be more comfortable without the midday sun, but they are not cool. This is less suited to travellers who want long, active days outdoors with a refreshing break after sunset.

Expect rain to affect your plans

The catch is frequent rain and limited sunshine. About 22 days have at least 1 mm of rain, while sunshine averages only 2.7 hours out of 12.4 hours of daylight. That makes May a poor match for a holiday built around dependable sunbathing. It does not mean every wet day is a washout, but you should leave room to rearrange outdoor time rather than count on a fixed dry window. Pack a light waterproof and keep something dry to change into. If grey skies would spoil the trip for you, the combination of heat and wet weather is a good reason to consider another month.

Warm sea, but not much cooling off

The sea is a clear reason to consider May if you like very warm water. At an average of 30.5°C, it is comfortably warm for swimming, though it will not provide much of a cool contrast to the air. That suits leisurely dips better than travellers seeking a bracing swim to shake off the heat. Water temperature alone does not tell you about conditions on the day, so follow local swimming advice rather than treating warmth as a safety guarantee. Overall, May works best for a relaxed pace, with swimming when conditions allow and sheltered breaks when the weather turns wet.