Koh Samui in May: Weather, the Regatta and Travel Tips
Koh Samui in May is the warmest month of the year — air and sea temperatures both peak just as the island slides into its first low season. Short answer: May is worth it especially if lower prices and smaller crowds matter more to you than a guarantee of fully dry weather. Rain days pick up noticeably, but the real rainy season is still a long way off.
At a glance
| Metric | Value |
|---|---|
| Air temperature avg | 33°C by day / 26°C at night |
| Rainfall | around 130mm over roughly 11 rain days |
| Sea temperature | around 31°C (warmest month of the year) |
| Sunshine hours/day | approx. 6 |
| Sunrise/sunset (mid-month) | approx. 5:59am / 6:33pm |
| Season | start of the first low season |
| Crowds / prices | noticeably cheaper than high-season April, except during the late-May Regatta |
We couldn't find a reliably sourced figure for daily sunshine hours on Koh Samui, so we've deliberately left it out rather than guess.
Weather in May
May is on average the hottest month and, with around 11 rain days, noticeably wetter than April. Expect intense but mostly afternoon-or-evening showers and thunderstorms — mornings are still almost always sunny. Koh Samui's real core rainy season, with days of sustained rain, doesn't arrive until October.
Wind and sea are generally still calm on both coasts in May. The southwest monsoon, which picks up from around May, mainly affects the Andaman coast around Phuket and Krabi; the northeast monsoon that brings Koh Samui's actual rainy season doesn't kick in until autumn. Boat trips and inter-island ferries largely run as normal in May, though the odd short-notice cancellation for an isolated thunderstorm is possible.
What to do in May — and what not to
The day trip to Koh Nang Yuan and Koh Tao still benefits from calm seas and good snorkelling visibility in May:
from 1,850 THBDay trip snorkeling Koh Tao & Koh Nang Yuan4.78 from 169 reviews
If you'd rather go somewhere quieter and less touristy, the longtail boat trip to the secret islands in the south is a good match — small groups of 10 to 12 people, a mangrove forest and coral reefs that are still in good shape:
from 2,300 THBKoh Samui Tours - Secret islands boat trip4.97 from 117 reviews
Beaches on both coasts remain perfectly usable in May. If you do get caught in an afternoon shower, a shopping mall, an air-conditioned temple tour, or an indoor activity will bridge the gap easily.
Festivals and events in May
The Koh Samui Regatta is the island's biggest home-grown event of the month: the 23rd edition ran from 23 to 30 May 2026 off Chaweng Beach, with evening races and a big closing party. 2027 dates weren't published at the time of checking, but expect a similar late-May window — Chaweng books up accordingly during the event.
Also traditionally held in May: the Samui Bike Week at Chaweng Lake, last held on 2–3 May 2025, with motorbike shows and an island parade.
Worth planning around: Visakha Bucha, one of the most significant Buddhist holidays, falls on 31 May in 2026 and 20 May in 2027. A nationwide 24-hour alcohol sales ban applies at shops, restaurants and most bars on this day — plan accordingly if you were hoping for evening drinks. More on Thailand's major festivals throughout the year in our Top 10 Events in Thailand.
Fruit in season
May marks the start of rambutan season (May to September), while durian and mangosteen are right in the middle of their main season. Lychee is still around through June too. Combined with mango season running to July, May is one of the fruit-richest months of the year. Details on every variety in our Top 10 Fruits in Thailand.
Frequently asked questions
Is May a good time to visit Koh Samui?
Yes, especially for the price: the first low season begins, hotels and activities get cheaper, and the sea stays mostly calm. Expect more rain days than April, and plan around the late-May Regatta if it interests you — or avoid it if you'd rather have Chaweng quiet.
How hot is Koh Samui in May?
Daytime averages around 33°C, with nights barely dropping below 26°C. The sea hits its annual high of around 31°C.
Does the rainy season start in May on Koh Samui?
Not really, not yet. With around 11 rain days, May is wetter than April, but the core rainy season with the most rain days falls between October and December.
What is the Koh Samui Regatta?
An annual sailing event off Chaweng Beach, traditionally held in late May, with evening races and a big closing celebration. The 23rd edition in 2026 ran from 23 to 30 May.
For the month before, see Koh Samui in April, and for what comes next, see Koh Samui in June. The full climate table with the monsoon explanation is in Koh Samui weather and the truth about the rainy season, and a year-round overview is in Best time to visit Koh Samui.
Keep reading
Koh Samui in April: Weather, Songkran and Travel Tips
Koh Samui in April: temperature, rain days, sea temperature, the Songkran date, fruit in season and which boat trips work best.
Koh Samui in August: Weather, Sea and What's On
Koh Samui in August: temperatures, sea temperature, rain days, crowds and prices in the "Big Season", plus festivals, fruit season and tours.
Koh Samui in December: Weather, Holidays and What to Do
Rains ease off, Ang Thong closed until 15 Dec, and Christmas/New Year as the priciest week: the travel planner for Koh Samui in December.