Koh Samui in May: 32.9°C highs, the hottest month, with 130 mm of rain over 11 days and a 30°C sea. Low season begins, so prices ease. Full weather guide.
Is May a Good Time to Visit Koh Samui?
Koh Samui weather in May is the hottest of the year and turning wet: a 32.9°C average high, 25.7°C lows, a 30.0°C sea and 130 mm of rain over 11 days. Yes, May is a good time to visit if you can handle heat and the odd afternoon downpour. Rain returns as the southwest monsoon arrives, but 130 mm is still under a third of November’s 445 mm, and May is where low season begins: hotel rates ease, beaches thin out and restaurant tables open up.
Key takeaways
- 32.9°C average high is the hottest month of the year: warmer than June (32.7°C) and April (31.8°C)
- 130 mm of rain over 11 days, up from April’s 85 mm over 6 days, still under a third of November’s 445 mm
- Sea holds at 30.0°C; the east coast (Chaweng, Lamai, Bophut) generally stays calmer than the west
- Low season begins: hotel rates ease and beaches and restaurants empty out
- Waterfalls start recovering after the dry season
- Honest caveat: humidity is high and 6.0 sun hours a day means more cloud than March’s 7.5
Koh Samui Weather in May: The Numbers
May sits at the hinge between the hot, dry season and the green season. The figures below are Climates to Travel’s 1991-2020 climate normals, cross-checked against the Thai Meteorological Department (Sep 2026), and match the data in this site’s best time to visit guide.
| Metric | April | May | June |
|---|---|---|---|
| Avg high | 31.8°C | 32.9°C | 32.7°C |
| Avg low | 26.0°C | 25.7°C | 25.3°C |
| Rainfall | 85 mm | 130 mm | 135 mm |
| Rain days | 6 | 11 | 10 |
| Sunshine | 6.5 h/day | 6.0 h/day | 5.0 h/day |
| Sea temperature | 30.0°C | 30.0°C | 30.0°C |
May edges past April on heat while nearly doubling its rain days, yet keeps more sunshine than June. The mean temperature for the month is 29.3°C. No other month pairs the year’s highest average high with the start of the wet pattern that runs through summer.
What Is the Temperature in Koh Samui in May?
Highs average 32.9°C, lows 25.7°C and the month’s mean is 29.3°C, so May is the hottest month on the calendar, a degree above April and narrowly ahead of June’s 32.7°C. Heat builds through the month as humidity climbs ahead of the monsoon, and it shows first on exposed activities: a temple staircase at midday, a viewpoint hike, a long walk along Chaweng’s beach road without shade. This is the peak of the hot season that starts building in March.
Keep strenuous plans to early morning or late afternoon, drink more water than feels necessary, and treat midday as pool time. The sea is a warm 30.0°C, so it offers no relief from the heat, only from the sun.
Is May a Rainy Month in Koh Samui?
Moderately. May’s 130 mm across 11 rain days is the real return of regular showers after April’s 85 mm over 6 days, as the southwest monsoon starts reaching the Gulf side. Each rain day averages roughly 12 mm: a shorter afternoon shower most days rather than one big storm. Sunshine drops to 6.0 hours a day, cloudier than March’s 7.5, so pack for daily cloud and a real chance of rain, not the wall-to-wall sun of February or March.
The rainiest month on Koh Samui is November, at about 445 mm over 16 days, followed by October at 295 mm (see the rainy season guide). May is nowhere near that regime. It is also not the Thailand-wide monsoon picture: the southwest monsoon hits the Andaman coast (Phuket, Krabi) harder, while Samui, on the Gulf side, stays comparatively dry through the summer months per the summer guide. If you want to avoid rain, the months to skip are October and November, not May.
Best Beaches and Sea Conditions in May
The sea holds at 30.0°C in May, tied with April and June for the warmest water of the year, but the calmest coast changes as the wind swings southwest. Per the sea conditions by month guide, the north and east coasts (Chaweng, Lamai and Bophut) stay “calm” through May, while the south and west coasts, including Lipa Noi, Taling Ngam and Nathon, pick up afternoon chop.
For swimming or a flat afternoon, the east coast is the safer bet; west-facing beaches are swimmable but wavier by midday. Snorkeling trips and day trips to Ang Thong still run reliably: real disruption to boat schedules doesn’t arrive until October-November.
Waterfalls, Hiking and Temples in May
Koh Samui’s waterfalls start a genuine recovery in May as returning rain recharges the interior. Na Muang Waterfalls and Hin Lad Waterfall won’t be at their fullest until the October-January peak flow, but May’s showers are the first real step up from March and April’s thin trickle.
Hiking the Na Muang Waterfalls Trail and Hin Lad Waterfall Trail is more demanding in May’s heat, so start early and expect a slipperier path than in April. Temples carry the same caveat: Wat Phra Yai and the hillside Secret Buddha Garden involve open stone and stairs that get uncomfortably hot by mid-morning.
Events in May
May’s fixed date is the Samui Regatta, a multi-day sailing event based at Chaweng Beach running 23-30 May 2026 with racing, parties and a closing awards night, per the best time to visit guide. It pushes Chaweng-area rates up for that week: booking elsewhere, or outside those dates, avoids the bump.
Otherwise May is quiet, with Koh Phangan’s monthly Full Moon Party on its usual lunar schedule; confirm the exact date before booking a transfer.
Koh Samui in May: Prices, Crowds and Low Season
May is where Koh Samui’s low season genuinely starts. Per the shoulder season guide, villa and hotel rates in the May-to-early-June window typically run 10-25% below baseline, a real drop from April’s near-baseline, Songkran-adjacent rates. The best time to visit guide rates May’s crowd and price level “medium,” a step down from “high” through March and April.
The visible sign is empty tables: restaurants and bars packed in February have room to walk in, and beaches feel noticeably less crowded. Anyone chasing the lowest prices should still look to September-October, but May is the first month where value is the honest reason to visit. The trip cost guide and when to book guide help with planning.
What to Pack for Koh Samui in May
Pack for heat and humidity first, rain second: lightweight breathable clothing, reef-safe sunscreen and a hat for the year’s hottest sun, plus a packable rain jacket for one of May’s 11 rain days. Swimwear and water shoes cover beach days and the rockier stretches around Ang Thong, and a dry bag is worth carrying on any boat trip. Quick-dry fabrics beat anything heavy in May’s humidity. The packing list guide covers the rest of a standard Koh Samui kit.
April, May or June: Which Should You Pick?
All three sit inside Koh Samui’s hot-to-green-season stretch, but each trades differently. April is the driest of the three (85 mm) and just under May’s heat: the better pick for minimal rain risk. May is the hottest month and the first with regular rain, but it is also where low-season prices start and waterfalls begin to recover. June cools very slightly while adding a touch more rain (135 mm). For a mid-year trip see also July and August. Most travelers who want a cheaper trip and can live with an afternoon shower will like May; April suits the driest option. March and the rest of the year are in the best time to visit guide.
FAQ
What is the weather like in Koh Samui in May?
Hot, humid and increasingly showery: 32.9°C highs, 25.7°C lows, 6.0 sun hours a day, 130 mm of rain over 11 days and a 30.0°C sea, per Climates to Travel’s 1991-2020 normals. Most rain falls as a short afternoon shower, not an all-day washout.
Is May a good month to visit Koh Samui?
Yes, for travelers who want lower prices and can handle heat. May is the hottest month at 32.9°C, with rain returning at 130 mm over 11 days, well under November’s 445 mm peak. Low season begins, so hotel rates ease and beaches and restaurants empty out.
Is May a rainy month in Thailand?
On the Andaman coast (Phuket, Krabi) the southwest monsoon is arriving, but Koh Samui sits on the Gulf side and stays comparatively dry. Samui’s May rain is 130 mm over 11 days, against 445 mm in November. See the rainy season guide.
What is the rainiest month in Koh Samui?
November, at about 445 mm over 16 rain days, followed by October at 295 mm. May’s 130 mm is under a third of November’s total, and December (265 mm) is the third wettest of the year.
Which months should you avoid Koh Samui?
For rain, October and November: 295 mm and 445 mm, both over 16 rain days, with disrupted boat schedules and rougher seas on the east coast. May is far milder, though it is the hottest month, so heat-sensitive travelers may prefer January-March.
Is May the hottest month on Koh Samui?
Yes. May’s 32.9°C average high is the highest of the year, a degree above April’s 31.8°C and narrowly ahead of June’s 32.7°C. Humidity builds with the heat, so shadeless activities like temple stairs and viewpoint hikes feel more demanding than in the dry season.
Is the sea warm enough to swim in Koh Samui in May?
Yes. May’s sea temperature of 30.0°C ties April and June for the warmest water of the year. The sea conditions by month guide rates the north and east coasts (Chaweng, Lamai and Bophut) “calm” in May, while the south and west coasts pick up afternoon chop.
Are hotel prices lower in Koh Samui in May?
Yes. Per the shoulder season guide, May-to-early-June rates typically run 10-25% below baseline, a real drop from March and April. The Samui Regatta (23-30 May 2026) briefly lifts Chaweng-area rates, so booking outside that week keeps the full discount.
Are Koh Samui’s waterfalls worth visiting in May?
They are improving but not yet at their best. Na Muang and Hin Lad start recharging as May’s rain arrives, a step up from April’s near-dry flow, though the fullest water comes October through January.
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