| Month | Mean, °C | Mean max, °C | Mean min, °C | Rain, mm | Rain days | Humidity, % |
|---|---|---|---|---|---|---|
| January | 26.9 | 29.0 | 24.2 | 126.8 | 11.1 | 83 |
| February | 27.4 | 29.4 | 24.9 | 64.3 | 5.6 | 81 |
| March | 28.2 | 30.4 | 25.4 | 115.8 | 5.9 | 81 |
| April | 29.1 | 31.8 | 26.0 | 82.9 | 8.1 | 81 |
| May | 29.2 | 32.9 | 25.7 | 131.6 | 14.0 | 80 |
| June | 28.8 | 32.7 | 25.3 | 133.6 | 13.1 | 78 |
| July | 28.5 | 32.3 | 25.1 | 117.0 | 14.3 | 78 |
| August | 28.4 | 32.4 | 25.1 | 102.2 | 14.5 | 78 |
| September | 28.0 | 31.9 | 24.8 | 117.5 | 15.0 | 80 |
| October | 27.4 | 30.6 | 24.3 | 294.9 | 19.3 | 85 |
| November | 27.1 | 29.7 | 24.1 | 444.6 | 18.7 | 86 |
| December | 26.8 | 29.3 | 24.0 | 263.1 | 14.2 | 83 |
| Year | 28.0 | 31.0 | 24.9 | 1,994.3 | 153.8 | 81 |
The table above is a climate normal: averages over 30 years, not a forecast for any given year. A single year can be drier or wetter. For planning a trip, the normal is more useful than last year’s weather: it shows what to expect in a typical month.
What the numbers show
Rain peaks at the end of the year. October, November and December bring 1,002.6 mm between them, half of Samui’s annual 1,994.3 mm (our calculation from the TMD table). November alone gets 444.6 mm. In those three months it rains on 14-19 days out of 30, and humidity rises to 83-86%. Mind the method: TMD counts a rain day whenever at least 0.1 mm falls, so short showers count too.
The rest of the year is moderate, not bone dry. From February to April Samui gets 64-116 mm a month with 6-8 rain days. From May to September it is 102-134 mm a month, with more rain days: 13-15.
Temperature barely changes. Monthly means stay between 26.8 °C (December) and 29.2 °C (May), and nights average 24-26 °C all year.
Read March with care. It has almost as few rain days as February (5.9 against 5.6) but nearly twice the rain on average (115.8 mm against 64.3). The highest one-day rainfall at the station for the whole of 1991-2020, 414.7 mm, also fell in March.
Koh Samui and Phuket month by month
Both columns are TMD normals for 1991-2020: the Ko Samui station (48 550) and the Phuket town station (48 564).
| Month | Samui, mm | Phuket, mm | Samui, rain days | Phuket, rain days |
|---|---|---|---|---|
| January | 126.8 | 49.9 | 11.1 | 5.6 |
| February | 64.3 | 24.0 | 5.6 | 3.4 |
| March | 115.8 | 80.7 | 5.9 | 7.5 |
| April | 82.9 | 135.1 | 8.1 | 11.7 |
| May | 131.6 | 236.9 | 14.0 | 18.9 |
| June | 133.6 | 249.4 | 13.1 | 18.6 |
| July | 117.0 | 240.2 | 14.3 | 19.1 |
| August | 102.2 | 308.7 | 14.5 | 19.5 |
| September | 117.5 | 350.4 | 15.0 | 21.7 |
| October | 294.9 | 336.0 | 19.3 | 22.7 |
| November | 444.6 | 178.2 | 18.7 | 15.9 |
| December | 263.1 | 82.0 | 14.2 | 10.3 |
| Year | 1,994.3 | 2,271.5 | 153.8 | 174.9 |
From November to March Samui gets more rain than Phuket, from April to October less. In November Samui gets 2.5 times as much (444.6 mm against 178.2), in December 3.2 times (263.1 against 82.0). In August it is the reverse: 102.2 mm on Samui, 308.7 mm on Phuket. October is wet on both islands.
Samui is also a little cooler in the daytime: an average high of 31.0 °C over the year against 32.8 °C at the Phuket town station, and 29.7 °C against 32.1 °C in November.
If you are choosing an island for November, the comparison with thunderstorm days and one-day records is in Phuket or Koh Samui in November. Phuket’s full table is on Phuket weather by month.
Koh Samui weather by season
Our grouping of the monthly TMD figures in the table above (Ko Samui station, 1991-2020).
| Period | Months | Mean max, °C | Rain a month, mm | Rain days a month | Humidity, % |
|---|---|---|---|---|---|
| Least rain | February-April | 29.4-31.8 | 64.3-115.8 | 5.6-8.1 | 81 |
| Moderate rain, hottest | May-September | 31.9-32.9 | 102.2-133.6 | 13.1-15.0 | 78-80 |
| Heaviest rain | October-December | 29.3-30.6 | 263.1-444.6 | 14.2-19.3 | 83-86 |
| Transition | January | 29.0 | 126.8 | 11.1 | 83 |
Why Samui’s rainy season is different
TMD describes the country’s weather as driven by two monsoons. The south-west monsoon blows from May to October and brings moisture from the ocean, with the heaviest rain on windward coasts. The north-east monsoon blows from October to February and is generally cool and dry for most of the country.
For the south TMD makes an exception: the region has an east coast and a west coast with two rainy periods. In the south-west monsoon heavy rain falls on the west side, in the north-east monsoon on the east side. Phuket is on the west side, on the Andaman Sea. Samui is in the Gulf of Thailand, on the east side, so its wettest months come at the end of the year.
The station data shows the switch. From November to April the prevailing wind on Samui is from the east (north-east to south-east), from May to October from the west.
Today’s forecast from TMD
The normals show a typical month, not the coming week. For the next seven days use the TMD 7-day forecast: it is given by region, and for Samui the relevant one is Southern (East Coast), with wind and wave height. On 26 October 2026, for example, it forecast scattered to fairly widespread thundershowers with isolated heavy to very heavy rain there for 26-28 October.
Method and limits
- Source: Thai Meteorological Department report Climatological Data of Thailand for 30 Year Period (1991-2020), printed page 71 (Ko Samui); Phuket figures from page 82. The report is published on a TMD subdomain.
- Period: 30 years, 1991-2020, the latest 30-year period for which TMD has published normals
- How we compiled it: figures copied from the report without recalculation. We checked monthly totals against the annual values and found no discrepancies.
- Rain day: at least 0.1 mm in 24 hours, counted from 07:00 to 07:00
- What is missing: sea temperature, waves and sunshine hours. The report has no sunshine observations for the Ko Samui station, and we found no primary normals for sea temperature around the island. Koh Phangan and Koh Tao have no stations of their own in the report.
- Next update: when TMD publishes new normals or at our April 2027 review
Getting there: Bangkok to Koh Samui. Neighbouring islands: Samui to Koh Phangan and Koh Tao.
The same month across the country: Thailand in November, Thailand in December.
FAQ
What is the best month for weather on Koh Samui?
By the TMD normals, February: the driest month with 64.3 mm of rain and 5.6 rain days. March and April also have few rain days (5.9 and 8.1), with 115.8 and 82.9 mm. April and May are the hottest months, with highs of 31.8 and 32.9 °C. The normals do not cover waves or sea temperature.
When is the rainy season on Koh Samui?
October to December: TMD's 1991-2020 normals give 263-445 mm a month and 14-19 rain days. These three months bring 1,002.6 mm, half of the annual total. TMD describes heavy rain on the east coast of southern Thailand during the north-east monsoon, which blows from October to February, and Samui is on that side.
What is the weather like on Koh Samui in November?
November is the wettest month of the year: 444.6 mm of rain on average, 18.7 rain days and humidity of 86%, the highest of the year. Highs are around 30 °C (29.7) and nights around 24 °C. The highest one-day rainfall recorded in November in 1991-2020 was 363.9 mm.
What is the weather like on Koh Samui in December?
Still wet: 263.1 mm of rain and 14.2 rain days. December is also the coolest month, with a mean of 26.8 °C and highs of 29.3 °C. Rain falls off in January (126.8 mm, 11.1 days) and reaches its low in February.
Is Koh Samui or Phuket better in November and December?
By rainfall, Phuket. In November Samui gets 444.6 mm and Phuket 178.2 mm, in December 263.1 mm against 82.0 mm (TMD 1991-2020). From April to October it is the other way round: in August, for example, Samui gets 102.2 mm and Phuket 308.7 mm.
When is the dry season on Koh Samui?
Samui has no long dry spell like Phuket's December to March. The least rain falls from February to April: 64-116 mm a month and 5.6-8.1 rain days. From May to September it is 102-134 mm a month, but with 13-15 rain days.
What is the hottest month on Koh Samui?
May: a mean of 29.2 °C and an average high of 32.9 °C. April is almost the same, with a mean of 29.1 °C. The highest temperature at the station in 1991-2020 was 38.0 °C, reached in April and in July.
Does it rain all day on Koh Samui in the rainy season?
The normals cannot tell you that. TMD counts any day with at least 0.1 mm as a rain day, so the 18.7 rain days in November include short showers. The report does not publish how many hours it rains.
Sources
- Thai Meteorological Department. Climatological Data of Thailand for 30 Year Period (1991-2020), printed page 71 (Ko Samui) and page 82 (Phuket)
- Thai Meteorological Department. Natural hazards in Thailand: climate and seasons, pages 2-4 (Thai)
- Thai Meteorological Department. 7-day weather forecast (regions)
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