Thailand Weather by Month and the Best Time to Travel

Rain arrives on Thailand's two coasts in different months, so check the forecast again before each island hop. A HelloRoam Thailand eSIM, set up at home before you fly, keeps the Thai Meteorological Department's forecasts a tap away once you land in Bangkok or Phuket.

Thailand has three seasons, and its two coasts keep different calendars

The Tourism Authority of Thailand (TAT) splits the year into three seasons: a hot season it calls summer, from March to May; a rainy or "green" season from May to October; and a cool season from November to February.

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Cool is relative. Bangkok's average high is still 31°C to 32°C from November to January. What drops is the rain and the overnight low, which averages 20.8°C in Bangkok in December and 15.3°C in Chiang Mai.

The rain also doesn't arrive everywhere at once. Bangkok, the north and the Andaman coast around Phuket get most of theirs from May to October. Ko Samui (often spelled Koh Samui) gets less rain than Phuket through mid-year, then takes its three wettest months from October to December.

For Canadians, that lines up neatly: Thailand's best weather falls in Canada's coldest months, from the December holidays into February. A March Break trip lands at the start of the hot season instead. Bangkok's average high reaches 33.7°C in March, and Phuket still averages only 5.8 rain days that month.

Month-by-month averages for Bangkok, Chiang Mai, Phuket and Ko Samui

The figures below are long-term averages from the Thai Meteorological Department (TMD), published on the World Meteorological Organization's World Weather Information Service, one page per station. Bangkok's averages cover 1961 to 1990. The WMO doesn't state the period for the other three, so treat them as a guide to the pattern, not a forecast.

Three things stand out. September is Bangkok's wettest month, with 344 mm of rain on average, more than any month on Ko Samui except November. Phuket has its fewest rain days in February, 3.0 on average. And Ko Samui's November average of 489.6 mm is the highest monthly total of all four stations.

The best months change from region to region

The worst month depends on where you go: September in Bangkok and on the Andaman coast, November on Ko Samui, and April in Bangkok and the north if heat wears you down.

The north has cool nights, then a smoky dry season

Chiang Mai is where the cool season actually feels cool, and Chiang Rai, further north, averages 12.2°C on January nights. A warm layer for early starts in the hills earns its space in your bag.

The dry months have a catch. Canada's travel advice for Thailand, updated 22 September 2026, says seasonal smog during the dry season is recurrent, and that agricultural burning can also affect air quality in northern provinces, including Chiang Mai. It tells travellers to monitor air pollution levels, especially those with respiratory ailments or another pre-existing medical condition, so check current readings before you head north in the dry months.

The Andaman and Gulf coasts swap places mid-year

From May to September, Phuket averages 19 to 23 rain days a month. Ko Samui averages 14 to 16 in the same months, with roughly a third to a half of Phuket's rainfall. If your only weeks off fall in July or August and you want a beach, the Gulf side is the drier bet. If you're weighing a summer beach trip against other parts of Southeast Asia, compare Bali's month-by-month weather before you book.

Then the calendar flips. From October to December, Ko Samui gets 209 to 490 mm of rain a month, while Phuket dries out to 8.3 rain days by December.

Island parks close on fixed dates every year

Thailand's Department of National Parks, Wildlife and Plant Conservation closes some islands and trails for part of every year, often because of weather and to give the sites time to recover. TAT shared the current closure list in January 2026, and for some trips these dates matter more than the rain: no amount of luck gets you onto the Similan Islands in June.

Ang Thong is the odd one out. The marine park shuts from 1 November to mid-December, in the same Gulf waters as Ko Samui, just as most of the country turns dry. Closure dates can change, so check with the park before you plan a day around it.

A rainy-season trip works if you plan around the gaps

May to October is wet, but not every day is. Even in September, Bangkok's wettest month, an average of 18 days bring rain, which leaves about 12 with little or none.

October is the hinge month: Chiang Mai's rain days fall to 11.6, Similan and Surin reopen on 15 October, and Ko Samui heads the other way, into its wettest stretch. Even after the reopening, the National Park Office list keeps Ko Tachai and some Similan dive sites, including Fantasy Reef and East of Eden, closed with no end date.

Canada's travel advice dates the rainy season from June to November, a month later than TAT does. It warns that seasonal flooding can make roads impassable and advises against jungle treks in the rainy season because of mudslides. Build a spare day into any overland leg, and keep island hops flexible.

The green months have their own draws. TAT's southern fruit calendar puts the widest choice of fruit around the middle of the year, and Salika durian from Phang Nga is typically in season from May to July.

April brings the peak heat and Songkran

April is the hottest month in Bangkok, with an average high of 34.9°C, and in Chiang Mai, at 36.1°C. It's also Songkran, the Thai New Year, when towns turn into water fights. TAT's Songkran family guide gave 13 to 15 April as the 2026 festival dates, and Global Affairs Canada asks drivers to be particularly vigilant over holiday periods such as Songkran.

Getting online when you land in Thailand

HelloRoam's Thailand plans run on AIS and on True Corporation's TrueMove H and dtac networks. Mobile data matters more than usual on a trip shaped by weather: park notices, ferry changes and rain radar all live on your phone.

Before you leave, check that your phone supports eSIM and is unlocked. Then install the eSIM before you fly, while you're still on home Wi-Fi, and switch it on when you land.

If you're unsure what plan size to pick, estimate your trip's data use first. If you'd planned to roam with your Canadian carrier, see how eSIM and roaming costs compare. And if you're spending a week at one resort with good Wi-Fi and no day trips, hotel Wi-Fi may be all you need.