Koh Samui Weather and Climate: How the Island's Seasons Work

Summary

Koh Samui sits in the Gulf of Thailand, not the Andaman Sea, so its seasons differ from Phuket. The driest months are roughly February to April, the hottest months are March to May, and the wettest period is roughly October to December, with November usually the wettest single month. Daytime temperatures stay in a narrow warm band of about 29-32°C all year, and the sea stays around 28-30°C.

Gulf vs Andaman coast

Thailand has two coasts with almost opposite rainy seasons. Phuket and the Andaman coast are wettest from about May to October. Koh Samui and the Gulf islands (Koh Phangan, Koh Tao) are partly sheltered from that southwest monsoon and instead get most of their rain from the northeast monsoon, roughly October to December. This means July and August are often better on Koh Samui than on Phuket, while November is often better on Phuket than on Koh Samui.

Seasons

• Name: Dry and cooler season · Months: about January to April · Description: Driest and calmest period. February and March are typically the driest and sunniest months. This is the most popular and most expensive time to visit.
• Name: Hot season · Months: about March to May · Description: Temperatures and humidity climb; April is usually the hottest month (daytime highs around 32-34°C). Songkran, the Thai New Year water festival, falls in mid-April.
• Name: Warm shoulder period · Months: about May to September · Description: Warm with short, heavy showers rather than all-day rain. The island is largely sheltered from the southwest monsoon, so this period is often drier and sunnier than the Andaman coast at the same time.
• Name: Rainy season (northeast monsoon) · Months: about October to December · Description: The wettest period. November is usually the wettest month. Rain can come as multi-day grey, wet spells rather than short showers, seas can be rough, and ferries or boat trips may be cancelled.

Months

• Month: January · Temperature range: 22-29°C (daytime high 29°C, night low 22°C) · Average rainfall (mm): 123 · Average rain days: 10 · Rainfall: moderate · Sea temperature: about 28°C · Sea conditions: Generally calm with good underwater visibility. · Crowds: high · Notes: Calm seas, lowest humidity, very popular. First ~10 days still busy and expensive after New Year; late January is a good value window. · Confidence: HIGH
• Month: February · Temperature range: 23-29°C (daytime high 29°C, night low 23°C) · Average rainfall (mm): 39 · Average rain days: 5 · Rainfall: very low · Sea temperature: about 28°C · Sea conditions: Often very calm and clear; among the best months for snorkelling. · Crowds: high · Notes: One of the driest months. Calm, clear water; good for snorkelling and boat trips. Chinese New Year (late Jan or Feb) brings a short demand spike. · Confidence: HIGH
• Month: March · Temperature range: 24-30°C (daytime high 30°C, night low 24°C) · Average rainfall (mm): 54 · Average rain days: 5 · Rainfall: very low · Sea temperature: about 29°C · Sea conditions: Calm and clear. · Crowds: high · Notes: Still dry; warming up. Often named the best all-round month: strong sun, calm sea, slightly lower prices than Dec-Jan. · Confidence: HIGH
• Month: April · Temperature range: 25-32°C (daytime high 32°C, night low 25°C) · Average rainfall (mm): 85 · Average rain days: 8 · Rainfall: low · Sea temperature: about 30°C · Sea conditions: Calm; warmest sea of the year. · Crowds: high (Songkran week) · Notes: Hottest month. Songkran water festival around 13-15 April. Short afternoon thunderstorms possible late in the month. · Confidence: HIGH
• Month: May · Temperature range: 25-32°C (daytime high 32°C, night low 25°C) · Average rainfall (mm): 172 · Average rain days: 17 · Rainfall: high · Sea temperature: about 30°C · Sea conditions: Mostly calm. · Crowds: moderate · Notes: Start of shoulder season. Mostly sunny mornings, short afternoon showers. Prices drop roughly 20-30% below peak; island is green. · Confidence: HIGH
• Month: June · Temperature range: 24-31°C (daytime high 31°C, night low 24°C) · Average rainfall (mm): 118 · Average rain days: 16 · Rainfall: moderate · Sea temperature: about 30°C · Sea conditions: Mostly calm; occasional choppy days. · Crowds: moderate · Notes: Warm, bright mornings with brief afternoon showers. Fewer crowds. Gulf coast stays drier than Phuket side at this time. · Confidence: MEDIUM
• Month: July · Temperature range: 24-31°C (daytime high 31°C, night low 24°C) · Average rainfall (mm): 146 · Average rain days: 17 · Rainfall: moderate · Sea temperature: about 29°C · Sea conditions: Mostly calm; occasional choppy days. · Crowds: moderate to high · Notes: European summer holiday wave. Generally drier than visitors expect; short showers, mostly calm sea. Book family accommodation early. · Confidence: MEDIUM
• Month: August · Temperature range: 24-31°C (daytime high 31°C, night low 24°C) · Average rainfall (mm): 137 · Average rain days: 17 · Rainfall: moderate · Sea temperature: about 29°C · Sea conditions: Mostly calm. · Crowds: high · Notes: Busy with European family holidays. Similar weather to July; brief showers, warm sea. · Confidence: MEDIUM
• Month: September · Temperature range: 24-31°C (daytime high 31°C, night low 24°C) · Average rainfall (mm): 141 · Average rain days: 17 · Rainfall: moderate · Sea temperature: about 29°C · Sea conditions: Becoming less consistent; some choppy days. · Crowds: low to moderate · Notes: Quieter and cheaper again. Rain picks up slowly through the month; the sea becomes less consistent. · Confidence: MEDIUM
• Month: October · Temperature range: 23-30°C (daytime high 30°C, night low 23°C) · Average rainfall (mm): 275 · Average rain days: 20 · Rainfall: very high · Sea temperature: about 29°C · Sea conditions: Rougher, especially on the east coast; boat trips may be cancelled. · Crowds: low · Notes: Rainy season starts properly. Rougher seas on the east coast; some boat trips and ferries cancelled on bad days. Lowest prices of the year. · Confidence: HIGH
• Month: November · Temperature range: 23-29°C (daytime high 29°C, night low 23°C) · Average rainfall (mm): 463 · Average rain days: 19 · Rainfall: very high · Sea temperature: about 28°C · Sea conditions: Often rough; the Gulf can be uncomfortable for several days at a time; ferry and boat-trip disruption is common. · Crowds: low · Notes: Usually the wettest month (long-term average over 400 mm). Multi-day rain possible; ferries and boat trips often disrupted. Ang Thong Marine Park usually closed from 1 Nov (2026 reopening date not yet confirmed). · Confidence: HIGH
• Month: December · Temperature range: 22-29°C (daytime high 29°C, night low 22°C) · Average rainfall (mm): 188 · Average rain days: 13 · Rainfall: high · Sea temperature: about 28°C · Sea conditions: Improving through the month; usually calm by Christmas week. · Crowds: building to high (Christmas-New Year) · Notes: Rain eases through the month; early December can still be wet. Weather usually turns good before Christmas, when crowds and peak prices arrive. · Confidence: HIGH

About the monthly data

Temperature, rainfall, rain-day and sea-temperature averages are long-term climate averages compiled by weather2travel from CRU (University of East Anglia), UK Met Office and KNMI datasets. Rainfall totals vary strongly between years; treat monthly figures as rough guides, not promises. Other travel sources quote different monthly rainfall figures (for example 345-490 mm for November), which shows how uncertain single-number averages are.