May is the month Thailand changes coasts. The southwest monsoon arrives on the Andaman side, the Similan and Surin islands close on 16 May until 14 October, and Phuket’s rain more than triples – while the Gulf islands stay dry, the north finally clears of smoke, and prices fall to their lowest of the year. If you know which coast to be on, May is one of the best-value months in Asia.
The monsoon split, which most guides get wrong
Thailand has two coasts on two different monsoon cycles, and treating the country as one rainy season is the single most common planning error.
- The Andaman coast – Phuket, Krabi, Railay, Koh Lanta, Koh Phi Phi – runs on the southwest monsoon, roughly May to October. May is when it starts.
- The Gulf coast – Koh Samui, Koh Phangan, Koh Tao – is sheltered from it and runs on the northeast monsoon instead, roughly October to December. Its dry season covers January to August, so May on the Gulf islands is excellent.
In numbers: Phuket goes from about 105 mm of rain in April to roughly 195 mm in May, with rain on most days. The Gulf islands see nothing comparable. If you want a May beach holiday in Thailand, go to Samui, not Phuket.
The national park closure
Similan and Surin close from 16 May to 14 October every year. This is a fixed annual closure imposed by Thailand’s Department of National Parks to let the reefs recover, it is not weather-dependent, and no operator can take you there during it. Several other Andaman marine parks apply seasonal restrictions over the same period, so check any specific island before booking.
If a liveaboard or a Similan day trip is the point of your trip, you need to be there before 15 May – or wait until the second half of October.
Weather, region by region
- Bangkok – about 34°C by day and 27°C at night, and the rain arrives properly: roughly 215 mm across twelve days, usually as an intense late-afternoon downpour that clears within the hour. It is hot, it is humid, and it is entirely manageable if you plan around the afternoon.
- Phuket and the Andaman – about 30°C, roughly 195 mm, rain on most days. Seas get rough and some longtail services stop; heed the red flags, because the Andaman rip currents in monsoon season kill swimmers every year.
- Koh Samui and the Gulf – still in the dry season, warm and comparatively settled.
- Chiang Mai and the north – about 34°C with roughly 170 mm of rain, and the crucial change: the rains clear the burning-season smoke. The mountains reappear, the countryside goes green, and the north becomes worth visiting again.
Red flags are not decorative
Worth stating plainly because it is a recurring cause of tourist deaths: during the southwest monsoon, Andaman beaches including Phuket’s Karon, Kata and Patong post red flags for rip currents. Swimming when the red flags are up is illegal on some beaches and dangerous on all of them. Lifeguard cover is not continuous. If the flags are red, do not go in.
What May is genuinely good for
Value, and the Gulf. Green-season rates on the Andaman side are the lowest of the year, and if you can accept afternoon rain the resorts are half-empty and the landscape is at its most lush. On the Gulf side you get dry-season weather at shoulder prices. And the north, newly clear, is at its greenest.
See how much a trip to Thailand costs and best islands in Thailand for choosing between the coasts.
Public holidays and the alcohol rule
- Saturday 1 May – Labour Day.
- Tuesday 4 May – Coronation Day.
- Visakha Bucha, the most important Buddhist holy day, falls in May – expected around 20 May in 2027, though the lunar dates are confirmed by the Thai Cabinet. On Buddhist holy days alcohol sales are banned nationally, with exceptions for hotels, international airport terminals, licensed entertainment venues and designated tourist areas. Shops and supermarkets will not sell it.
The May 2027 calendar
- Saturday 1 May – Labour Day; Tuesday 4 May – Coronation Day.
- Sunday 16 May – Similan and Surin islands close until 14 October.
- Around 20 May – Visakha Bucha expected; national alcohol sales ban applies.
- Through May – southwest monsoon establishes on the Andaman coast; the north clears of smoke.
- All month – the Gulf islands stay in their dry season.
Read alongside
- Thailand in April – Songkran and the last of the Similans.
- Thailand in June – green season proper, and the cheapest weeks.
- Best time to visit Thailand – all twelve months compared.
- Best islands in Thailand – which coast, and when.
- Koh Samui vs Phuket – the comparison that matters most in May.
- How much a trip to Thailand costs – green-season numbers.
- What to pack for Thailand – the wet-season list.
Frequently asked
Is May a good time to visit Thailand?
Yes, if you pick the right coast. The Gulf islands – Samui, Phangan, Koh Tao – are still in their dry season, the north has cleared of burning-season smoke, and prices are at their lowest of the year. The Andaman coast is entering its monsoon.
When do the Similan Islands close?
16 May, reopening on 14 October. It is a fixed annual closure by Thailand’s Department of National Parks for reef recovery, not a weather decision, and no operator can run trips during it.
Does it rain all day in Thailand in May?
No. The typical pattern is a heavy late-afternoon downpour that clears within an hour or two, not continuous rain. Bangkok gets around 215 mm across twelve days; Phuket around 195 mm on most days.
Should I go to Phuket or Koh Samui in May?
Koh Samui. The two coasts run on different monsoons: the Andaman side, including Phuket, gets the southwest monsoon from May to October, while the Gulf islands stay dry from January to August.
Is it safe to swim on Phuket beaches in May?
Only when the flags allow. The southwest monsoon produces dangerous rip currents on Andaman beaches, red flags go up regularly, lifeguard cover is not continuous and people drown every year. If the flag is red, stay out of the water.
Is the burning season over in May?
Yes, essentially. The rains arrive and clear the haze, the mountains reappear and the north turns green. May is when Chiang Mai becomes worth visiting again.
Is alcohol banned on any days in May?
Yes – Visakha Bucha, expected around 20 May 2027, is one of five Buddhist holy days with a national ban on alcohol sales. Hotels, international airport terminals, licensed venues and designated tourist areas are exempt. The Thai Cabinet confirms the lunar dates.
Is May cheap in Thailand?
It is among the cheapest months of the year, particularly on the Andaman coast where green-season rates apply. The Gulf islands sit at shoulder prices with dry-season weather, which is the best combination available in May.
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Written by
John Morrison is the founder and lead travel writer at Packzup. Over the past decade he has explored destinations across Africa, the Americas, Asia, Europe, the Middle East, and Oceania — always self-funded, never on a press trip.
