Koh Samui Weather August: 100 mm Rain, 32.4°C, Sea 29.5°C

Is August a Good Time to Visit Koh Samui?

Koh Samui weather in August is warm, humid and comparatively dry: a 32.4°C average high, 25.1°C lows, a 29.5°C sea and 100 mm of rain over 10 days, the lowest rainfall of the May-September stretch. Yes, August is worth visiting. The catch is not the weather but the calendar: European school holidays make it one of the busiest and priciest months of the middle of the year, so decent conditions come with peak-style crowds.

Key takeaways

• 100 mm rain over 10 days, less than July (115 mm), September (120 mm), May (130 mm) and June (135 mm)
• 32.4°C highs, 25.1°C lows, 29.5°C sea: swimming weather every day, rain or not
• 5.0 sun hours a day, the annual floor shared with June, July and September
• Rain falls as short afternoon squalls, so mornings are the reliable window
• East coast (Chaweng, Lamai, Bophut) is the calmer swim; the west coast picks up afternoon chop
• Busiest and most expensive month of the mid-year window, driven by holidays, not weather

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Koh Samui Weather in August: The Numbers

August is the driest month of Koh Samui’s southwest-monsoon stretch, but not the driest of the year: February holds that title with 65 mm over 4 days and 7.5 sun hours. The figures below are 1991-2020 climate normals from Climates to Travel, cross-checked against the Thai Meteorological Department (checked Sep 2026).

Sun hours are 5.0 a day in July, August and September, against 7.5 in February. The full 12-month table sits in the best time to visit guide.

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What Is the Temperature in Koh Samui in August?

Daytime highs average 32.4°C and nights hold at 25.1°C, so there is no cool part of the day, only a cooler one. The sea sits at 29.5°C on every coast, warmer than in December (28.5°C) or January (28.0°C). August feels almost identical to its neighbours: September is half a degree cooler at 31.9°C and July a tenth of a degree cooler at 32.3°C. Rain barely changes the heat; it mostly changes the humidity for an hour.

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Koh Samui August Rain: What a Rain Day Looks Like

August’s 10 rain days arrive as short, convective afternoon squalls, not sustained drizzle, so a fully rained-out day is rare. The usual pattern is a clear or partly cloudy morning, cloud building through midday, and a heavy 20-40 minute downpour some time in the afternoon before skies clear by evening. This is the same short-burst pattern the rainy season guide describes for October-December, at lower volume and frequency.

Front-load the day: book boat trips, beach time and outdoor plans for the morning, and treat early-to-mid afternoon as the window most likely to lose an hour.

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Is Thailand Too Rainy in August? Samui vs Phuket

Not on Koh Samui. The island sits on the Gulf of Thailand side, out of reach of the southwest monsoon that soaks the Andaman coast (Phuket, Krabi) in these months. The summer guide puts Samui at roughly 350 mm across June-August against about 725 mm for Phuket over the same three months. Samui’s own wet season is October-December, driven by the opposite (northeast) monsoon, and it peaks in November at 445 mm over 16 days: the rainiest month on the island. If you are choosing which months to avoid for rain, that is October and November, covered in the rainy season guide.

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Koh Samui Weather August to September: How Late August Feels

Late August runs on the same pattern as the rest of the month: warm 32.4°C days, a 29.5°C sea and short afternoon squalls. The step change comes later. September adds 20 mm of rain and one rain day over August (120 mm over 11 days), a modest shift, and the real cliff is October’s 295 mm over 16 days. A trip from the end of August into mid-September therefore stays in fairly settled weather, and the higher-risk window starts in the last week of September. See September and October for the month-by-month numbers.

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European Summer Meets Low Season: The Real Trade-Off

August’s honest trade-off is that it delivers the wet-season stretch’s best rainfall number while also being one of the busiest months, because European school summer holidays overlap it almost exactly. Hotel rates, villa prices and flights rise noticeably, and Chaweng and Bophut run at their busiest of the mid-year window, a step above July, per the shoulder season guide’s crowd-and-price framing.

The trade-off is two-sided. Weather-wise, August beats the true low season (September-November) that follows: less rain and calmer seas on most coasts. Price-wise, it sits well above the June low point and the September-October lull, without reaching the December-January peak. Travelers with flexible dates get a better weather-to-price ratio in May or June; those locked into European summer dates get decent weather, just not the cheapest version of it. The best time to visit hub has the full crowd and price index.

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Which Coast to Choose: East Stays Calmer, West Picks Up Chop

Pick the east coast for the calmer swim in August: the southwest monsoon blows onshore against the west coast, so Chaweng, Lamai and Bophut generally stay flatter, while Lipa Noi and Taling Ngam pick up afternoon wind and chop. The sea conditions by month guide and the east vs west coast comparison describe the same May-September pattern. Sea temperature never drops below 29°C on any coast, so the choice is about wave and wind comfort, not water temperature.

The west coast is not unswimmable in August, and its sunsets are still the island’s best. The trade-off is a choppier afternoon swim and a 25-45 minute drive from the airport, per the east-vs-west breakdown.

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Waterfalls Run Strong After the Wet Months

August is a good month for Koh Samui’s waterfalls: several weeks of May-through-August rain keep both main falls running well. Na Muang Waterfall and Hin Lad Waterfall carry solid flow into their pools, without the harder, trail-flooding rain of October-November that makes footing and photography difficult. It is one of the month’s clearest upsides against the driest months of the year, when both falls thin out considerably.

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Hiking in August: Humid, Slippery, Start Early

Trails are green and humid in August, and the rain that keeps the waterfalls flowing also leaves roots, rocks and packed earth slick underfoot, especially on shaded jungle routes like the Na Muang Waterfalls Trail and Hin Lad Waterfall Trail. Start before 9 am: mornings are usually dry and cooler, while afternoon humidity and the chance of a squall both build fast. Exposed viewpoint trails dry faster after a shower than jungle routes.

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What to Pack for August

Pack for heat and short rain: lightweight quick-dry clothing, reef-safe sunscreen, and a compact umbrella or packable rain jacket. A dry bag is worth carrying on any boat day trip or scooter ride, since squalls can arrive with little warning. See the day trips guide for which tours run most reliably in August. Box jellyfish risk stays elevated through August per the jellyfish guide, so check posted flags before swimming. The full kit is in the packing list; the trip cost guide covers August’s higher prices.

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FAQ

What is the weather like in Koh Samui in August?

Warm, humid and mostly sunny with short afternoon showers: 32.4°C highs, 25.1°C lows, 5.0 sun hours a day, 100 mm of rain over 10 days and a 29.5°C sea, per 1991-2020 normals from Climates to Travel. Rain arrives as squalls, so mornings are the reliable window.

Is August a good time to visit Koh Samui?

Yes. August is the driest month of Koh Samui’s May-September stretch, at 100 mm over 10 days, with 32.4°C highs and a 29.5°C sea. The trade-off is crowds and prices: European school holidays make it one of the busier, pricier months, regardless of the decent weather.

Does it rain a lot in Koh Samui in August?

Less than the months around it. August’s 100 mm across 10 rain days beats July’s 115 mm over 11 days and September’s 120 mm over 11 days, though it is still well above February’s 65 mm over 4 days. Most rain falls as short afternoon squalls, not all-day rain.

Is Thailand too rainy in August?

The Andaman coast (Phuket, Krabi) gets much more rain than the Gulf. Koh Samui logs roughly 350 mm across June-August against Phuket’s about 725 mm, per the summer guide, and August’s 100 mm is the lowest of Samui’s mid-year months.

What is the rainiest month in Koh Samui?

November, at about 445 mm over 16 rain days, followed by October at 295 mm. August’s 100 mm is less than a quarter of November’s total. See the rainy season guide for the full October-December picture.

Why is Koh Samui so busy in August if it is not the driest month?

Because European school summer holidays land on top of it, not because of the weather. Chaweng and Bophut run at their busiest of the mid-year window, and hotel and flight prices rise to match, even though February has better rainfall numbers.

Which beach is calmest in Koh Samui in August?

The east coast (Chaweng, Lamai and Bophut), since the southwest monsoon blows onshore against the west coast, leaving Lipa Noi and Taling Ngam choppier in the afternoons. Sea temperature holds at 29.5°C on every coast, so the choice is about wave comfort.

Are Koh Samui’s waterfalls running in August?

Yes, and August is a good window. Na Muang and Hin Lad both carry solid flow after weeks of May-through-August rain, without the trail-flooding downpours of October-November. Trails are humid and can be slippery, so start hikes before 9 am.