Thailand's rainy season (roughly mid-May to October in most of the country, October to December on Koh Samui's coast) is not a bad time to visit, but it is a different trip. Most days still have long dry spells and some sun. The real risks are rough seas that stop small boats, a few days of non-stop rain when a storm system passes, and flooding in September and October.
If you plan for those three things, the rainy season gives you green landscapes, fewer crowds and more choice of rooms. If your trip depends on one specific boat tour or dive day, or you have no slack in your schedule, it can go wrong. This page is about the practical side: what the rain looks like day to day, what gets cancelled, and how to build a trip that survives a wet week.
Our older article on the best time to visit Thailand explains the seasons in general terms. For month-by-month rainfall at each region, see Thailand's weather month by month.
The rainy season in numbers (TMD averages 1991–2020)
- Bangkok, wettest month
- September: 335.9 mm on 21.4 days with rain
- Bangkok sunshine
- About 130 hours in September, against 216 in January
- Chiang Mai, wettest month
- August: 223.3 mm on 20.9 days with rain
- Phuket, wettest months
- September and October (350.4 and 336.0 mm, Phuket town)
- Koh Samui, wettest month
- November (444.6 mm)
- Weather warnings
- Thai Meteorological Department, hotline 1182
What a rainy-season day actually looks like
The Thai Meteorological Department counts any day with at least 0.1 mm of rain as a "rainy day". By that measure, Bangkok has rain on 21 of 30 days in September. That sounds grim, but a day with a 40-minute downpour at 5 p.m. counts the same as a wet day. Sunshine records give a fairer picture: Bangkok averages about 130 hours of sun in September, roughly four hours a day, against about seven hours a day in January. Chiang Mai in August averages about 111 hours, or three to four hours a day.
In practice there are three kinds of wet weather:
- Thunderstorms. The most common pattern. A hot, humid morning builds into a short, very heavy storm, usually in the afternoon or evening. Bangkok averages more than 10 thunderstorm days in September. Streets can flood for an hour, then drain.
- Monsoon days. Grey skies and steady rain on and off all day, more common on the Andaman coast and in the north at the height of the season.
- Storm spells. When a low-pressure system or a stronger monsoon surge arrives, it can rain hard for two to four days in a row over large areas. These spells cause most floods and most boat cancellations.
The first two are easy to work around. Plan outdoor sightseeing for the morning, keep museums, markets, malls and massages for the afternoon, and carry a light rain jacket or a folding umbrella (7-Eleven sells cheap ones). The third type is the one to plan for.
Boats, ferries and island tours: what gets cancelled
On the islands, the sea matters more than the rain. When the monsoon strengthens, TMD issues warnings for each sea area. A typical warning from late September 2026 forecast waves of 2–3 meters in the upper Gulf and upper Andaman Sea, higher in thunderstorms, and told mariners that "small boats should keep ashore". The Nation reported two such warnings in September 2026 alone, covering 17–20 September and 25–27 September.
Those warnings hit the boats travelers use most: speedboats and longtails for snorkeling trips, island-hopping tours and short transfers. Larger ferries and car ferries usually keep running in moderate seas, but crossings get rough and slow. The UK Foreign Office warns that passenger boats between the mainland and islands have sunk or collided, and names rough seas, particularly during the monsoon, as one of the usual causes.
Some marine areas close completely for the season:
- Similan and Surin Islands (Phang Nga): closed to visitors every year from 15 May to 15 October.
- Maya Bay on Koh Phi Phi Le: closed from 1 August to 30 September each year; it reopened on 1 October 2026. Our page on visiting Maya Bay has the rules.
- Inland parks can close parts of their area too. Kaeng Krachan closes some zones for three months each rainy season and reopens them on 1 November, and Khao Yai National Park closed after flood damage on 25 September 2026 and only reopened, partly, on 6 October.
Leave a spare day between an island and your flight home. If a boat warning cancels your transfer from Koh Lipe, Koh Phi Phi or Koh Tao, the next boat may be a day later. A night in Phuket, Krabi, Surat Thani or Bangkok before an international flight costs far less than a missed connection.
If you are choosing islands for a rainy-season trip, the Gulf islands (Samui, Phangan, Tao) are usually calmer than the Andaman side from June to September. Our guide to the best time for the Thai islands compares them, and our page on ferries in Thailand covers operators and safety.
Floods: September and October are the risky months
The worst flooding usually comes late in the season, when the ground is already soaked and rivers are high. The latest reminder came in late September 2026. Heavy rain flooded parts of Bangkok and nearby provinces, and the Cabinet declared special public holidays on 28 and 29 September in Bangkok, Nonthaburi, Pathum Thani and Samut Prakan. TAT's visitor updates reported disruption on the State Railway's Northern Line and the Red Line on 26 and 27 September, and Thai Airways cancelled selected flights on 29 September. By the afternoon of 29 September TAT said conditions had improved and the main sights, both Bangkok airports, the BTS, MRT and Airport Rail Link were operating.
Two lessons follow. First, a flood in Thailand is usually local and short, not a reason to cancel a whole trip. Second, it can still cost you a day or two, so keep your plans flexible in September and October. The UK Foreign Office adds that heavy rain can cause flash floods and landslides, particularly in the north, the northeast and mountainous areas, and that lakes, caves and waterfalls "are particularly at risk of dangerous flash flooding".
Do not enter caves or swim under waterfalls after heavy rain. Water levels rise within minutes. River and waterfall currents are at their strongest in the rainy season, and flash floods in caves have trapped people for days. If a park ranger closes a trail or a pool, the closure is there for a reason.
Health in the wet months
Mosquitoes breed in standing water, and dengue cases in Thailand tend to rise in the rainy season; the World Health Organization has linked recent surges to the monsoon. Use repellent during the day as well as in the evening, since the mosquitoes that spread dengue bite in daylight. Our pages on dengue fever and avoiding mosquito bites go into detail.
Floodwater is the other risk. The US CDC lists leptospirosis and melioidosis for Thailand, both linked to contact with contaminated fresh water, mud or soil, especially after heavy rain. Avoid wading through floodwater in sandals, cover cuts, and see a doctor if you develop a fever after contact with floodwater.
At sea, the Foreign Office notes that jellyfish come close to shore particularly in the rainy season, from May to October, and that rip currents drown swimmers on monsoon beaches. Swim only where lifeguards are on duty and never when a red flag is up; see our guide to rip currents and beach flags.
Roads, scooters and getting around
Wet roads are the hidden danger of the season. Painted road markings, metal plates and mud from building sites become slippery, and drainage can turn a street into a river in minutes. If you have never ridden a scooter in the rain, Thailand's rainy season is not the place to learn; our guide to riding a motorbike safely explains the basics. In the cities, taxis and ride-hailing cars are harder to find in a downpour, so leave early for anything with a fixed time.
Mountain roads in the north, such as the routes to Pai and Mae Hong Son, can be hit by landslides after heavy rain. Long-distance buses and trains generally run, but allow for delays.
Prices and crowds: what low season really saves
Low season is real in the beach areas: many hotels on Phuket, Krabi and Koh Phi Phi cut their rates between May and October, and popular places are noticeably quieter. We do not have a reliable official source for how large those discounts are, and they vary by hotel and year, so compare the same hotel's rates for different months rather than trusting a general percentage. Food, transport and entrance fees stay the same all year.
Do not assume every region is in low season at the same time. From June to August the Gulf islands are drier than the Andaman coast, so they attract the travelers who would otherwise go to Phuket, and their prices do not necessarily drop. Long weekends around Thai public holidays, such as Asanha Bucha in July or the King's Birthday on 28 July, can also bring Thai travelers to the coast in large numbers, rain or not; our list of Thai public holidays has the dates. A travel insurance policy that covers weather-related delays is worth checking before you book non-refundable rates.
Frequently asked questions
Does it rain all day in Thailand's rainy season?
Usually not. Most rain falls in short, heavy afternoon or evening storms, and Bangkok still averages about four hours of sunshine a day in September, its wettest month. Spells of two to four days of steady rain do happen when a storm system passes.
Are ferries cancelled in the rainy season?
Sometimes. When the Thai Meteorological Department warns of waves of 2–3 meters, it tells small boats to stay ashore, which stops many speedboat transfers and island tours. Larger ferries usually run in moderate seas. Keep a spare day before an international flight.
Which part of Thailand is driest in the rainy season?
Between June and August the Gulf islands (Koh Samui, Koh Phangan, Koh Tao) and Hua Hin are the driest beach areas. Samui averages 102–134 mm a month in those months, against 240–309 mm in Phuket town.
Is Bangkok likely to flood when I visit?
Short street flooding after storms is common from May to October and usually drains within hours. Serious flooding is rarer and most likely in September and October, as in late September 2026. Check TMD warnings and TAT updates before travel days.
Is the rainy season cheaper?
Accommodation in the Andaman beach areas is usually cheaper between May and October, and crowds are smaller. How much cheaper varies by hotel, so compare the same hotel across months. Flights, food and entrance fees do not follow the season in the same way.
Sources
- Thai Meteorological Department – Climatological Data of Thailand 1991–2020 (checked October 2026)
- Thai Meteorological Department – Strong wind-waves warning No. 11 (127/2026) (checked October 2026)
- Nation Thailand – TMD warns of heavy rain and 2–3 m waves, 25–27 September 2026 (checked October 2026)
- Tourism Authority of Thailand – Weather and travel conditions in Bangkok, visitor information (checked October 2026)
- Tourism Authority of Thailand – Special public holidays on 28–29 September 2026 (checked October 2026)
- UK Foreign, Commonwealth and Development Office – Thailand: safety and security (checked October 2026)
- US Centers for Disease Control and Prevention – Thailand traveler view (checked October 2026)
- Department of National Parks – Maya Bay seasonal closure 2026 (checked October 2026)
- Nation Thailand – Similan and Surin islands close for the monsoon season (checked October 2026)