- The southwest monsoon hits the Andaman coast (Phuket, Krabi, Khao Lak) from roughly May to October, peaking in September.
- The Gulf coast islands (Koh Samui, Koh Phangan, Koh Tao) dodge that pattern and get their wettest weather from October to December, peaking in November.
- Bangkok and Chiang Mai follow the mainland’s rainy season, roughly mid-May through October.
- Rain usually falls in short, heavy bursts rather than all-day downpours — mornings are often clear.
- Low season means fewer crowds and cheaper hotels and flights, but also rougher Andaman seas and a higher dengue risk.
When Is Thailand’s Monsoon Season, Officially?
Thailand’s weather runs on three seasons: hot (March to May), rainy (June to October), and cool (November to February). The Thai Meteorological Department declared the 2026 rainy season officially underway from 15 May, when the southwest monsoon set in over the Andaman Sea, the mainland, and the Gulf of Thailand.
That national timeline is only half the story, though. The south of the country actually runs on two separate monsoon systems that hit opposite coasts at different times of year.
The Real Story: Thailand Has Two Monsoons, Not One
The Andaman coast — Phuket, Krabi, Khao Lak — faces the Indian Ocean and takes the full force of the southwest monsoon from May to October. The Gulf coast — Koh Samui, Koh Phangan, Koh Tao — faces the other direction and is sheltered from that same wind, so it stays comparatively dry through the same months.
Instead, the Gulf coast gets soaked later by the northeast monsoon, which picks up moisture crossing the Gulf of Thailand and dumps it on Koh Samui from October to December. The chart below shows exactly how the two coasts trade places across the year.
Koh Samui (Gulf coast)
Phuket’s rain builds steadily from May, tops out around 318mm in September, and only really eases off in November and December. Koh Samui does almost the opposite — it stays under 140mm every month from January through September, then jumps to roughly 295mm in October and a soaked 445mm in November, its wettest month by far.
Phuket, Krabi, and the Andaman Coast: When It Rains
Phuket gets progressively wetter from May, with June through August bringing regular afternoon storms and September as the true peak. Krabi and Khao Lak follow a near-identical pattern, since they sit on the same stretch of Andaman coastline.
A strengthening southwest monsoon regularly pushes Andaman wave heights to 2–3 metres, and Thai authorities have repeatedly warned small boats to stay ashore during the roughest spells.
- Ferries: Routes between Phuket, Phi Phi, Koh Lanta, and Koh Lipe are frequently cancelled or delayed when seas turn rough, especially June through September.
- Diving and snorkeling: Some Andaman dive operators pause trips to exposed sites during the roughest weeks and shift to more sheltered spots instead.
- Best light window: Mornings are usually clear even in peak season, so plan boat trips and beach time before early afternoon.
Koh Samui, Koh Phangan, and Koh Tao: The Gulf Coast’s Opposite Season
Koh Samui and its neighbours Koh Phangan and Koh Tao are the reason this whole two-coast trick works for trip planning. While the Andaman coast is in its wettest stretch, the Gulf islands are usually enjoying some of their driest, sunniest months of the year.
That flips completely by October, when the northeast monsoon takes over the Gulf of Thailand.
November is Koh Samui’s soggiest month, with October and December not far behind, so a beach trip timed for July or August makes far more sense here than a November one.
Bangkok, Chiang Mai, and the Mainland
Bangkok follows the national rainy-season calendar, with September its wettest month at roughly 333mm and short, intense afternoon thunderstorms typical through the summer.
Flash flooding on low-lying streets is common after the heaviest downpours, so it pays to check forecasts before a full day of sightseeing.
Chiang Mai and the north see a shorter, slightly later rainy season, running roughly mid-May to October and peaking in August. Waterfalls around Chiang Mai and Chiang Rai are at their most dramatic during these wetter months, which is one of the season’s genuine upsides for nature-focused travellers.
Where to Go, Month by Month
Should You Still Visit? The Pros and Cons of Green Season Travel
Thailand’s rainy months are sometimes marketed as “green season” for good reason.
Pros:
- Fewer crowds: Popular sites and beaches are noticeably quieter outside the November–February peak.
- Lower prices: Hotels across Thailand commonly cut rates through the low season, and Bangkok Post has reported destinations like Phuket discounting rooms by as much as 50% even as occupancy only drops by a third.
- Lush scenery: Rice paddies turn vivid green and waterfalls run at full flow, especially in the north.
- Short storms, not washouts: Rain typically arrives as an intense burst for an hour or two, often in the afternoon, rather than an all-day event.
Cons:
- Rough Andaman seas: Ferry cancellations and choppy crossings are common on the west coast during peak monsoon weeks.
- Flood risk: Bangkok and low-lying areas can see localised flash flooding after heavy downpours.
- Dengue risk: Standing water gives mosquitoes more places to breed, so bite prevention matters more during the rainy months. The CDC advises all travellers to Thailand to take steps to avoid mosquito bites, since dengue has no routine travel vaccine.
- Some closures: A handful of dive shops and boat operators scale back or pause trips to exposed Andaman sites during the roughest weeks.
A hotel with a pool and covered walkways makes a rainy stretch far more comfortable, and that’s easy to shop for on the Phuket hotel or Bangkok hotel listings before you book.
Practical Tips for Visiting Thailand During Monsoon Season
- Build in flexibility: Leave a buffer day around any island-hopping ferry, especially on the Andaman coast.
- Pack light rain gear: A compact umbrella and a dry bag for electronics cover most storms.
- Protect against mosquitoes: Repellent, covered clothing in the evenings, and accommodation with screened windows all help.
- Check marine warnings: The Thai Meteorological Department posts small-craft and rough-sea advisories before island trips during storm surges.
- Pick the right coast for the month: Use the chart above to match your travel dates to whichever coast is driest that month.
If you’d rather build a whole family itinerary around this season instead of just planning around the rain, Wego’s guide to Thailand’s green season covers what to actually do once you’re there.
Frequently Asked Questions
When is Thailand’s monsoon season?
Nationally, Thailand’s rainy season runs roughly June to October, officially starting mid-May. But the Andaman coast (Phuket, Krabi) is wettest May to October, peaking in September, while the Gulf coast islands (Koh Samui) are wettest October to December, peaking in November.
Why does it rain in Phuket but stay sunny in Koh Samui, or the other way round?
The two coasts face opposite monsoon winds. Phuket and the Andaman coast take the brunt of the southwest monsoon from May to October, while Koh Samui and the Gulf coast are sheltered from that wind but exposed to the northeast monsoon instead, which peaks from October to December.
Is it worth visiting Thailand during monsoon season?
Yes, for most travellers. Rain usually falls in short, heavy bursts rather than all day, and you get lower prices, thinner crowds, and lush green scenery in exchange for planning around the wetter coast.
Do ferries to the islands still run during the rainy season?
Usually yes, but Andaman routes (Phuket–Phi Phi, Phuket–Koh Lanta, and onward to Koh Lipe) are the most prone to cancellation or delay when seas turn rough, mainly June to September. Build a spare day into any island-hopping plan on that coast.
Is dengue a real risk during Thailand’s rainy season?
It’s a genuine year-round risk in Thailand that rises with the rains, since standing water gives mosquitoes more places to breed. The CDC recommends all travellers take standard bug-bite precautions, as there’s no routine travel vaccine against dengue.
What’s the best month to avoid rain almost everywhere in Thailand?
December is close to a sweet spot for the mainland and the Andaman coast, both drying out by then, though Koh Samui is still finishing its own wet stretch. For all-round dry weather, November through February is Thailand’s best window.
Plan Around the Rain, Not Despite It
Thailand’s monsoon season isn’t one blanket “rainy time” to avoid — it’s two separate weather systems that rarely soak the whole country at once. Match your coast to your month, using the chart above as a quick reference, and compare flights and hotels for your dates on Wego.
Sources
- Thai Meteorological Department — Monthly Weather Summary (2026 rainy season onset)
- weather-and-climate.com — Phuket Average Monthly Rainfall
- climatestotravel.com — Ko Samui Climate and Monthly Averages
- climatestotravel.com — Phuket Climate (Andaman vs Gulf coast rainfall)
- weather-and-climate.com — Bangkok Average Monthly Rainfall
- weather-and-climate.com — Chiang Mai Average Monthly Rainfall
- The Nation Thailand — Monsoon Surge to Bring Heavier Rain and Rough Seas to Thailand
- CDC Travelers’ Health — Thailand
- Bangkok Post — Hotel Room Rates Plunge Too Low During Low Season
Disclaimer: We’ve worked hard to make sure the information in this article is as accurate and up to date as possible at the time of publishing. Travel details such as prices, schedules, visa rules, and entry requirements can change at short notice, so it remains your responsibility to verify the current details with official sources before you book or travel. Wego accepts no liability for any loss or inconvenience arising from the use of this content.
