When to Go to Thailand: the Seasons Coast by Coast, From the Weather Office's Own Figures

Asking when to visit Thailand is really asking which part of it. The Andaman coast and the Gulf islands get their heaviest rain months apart, the north has a smoke season the beaches do not, and several marine parks close outright for part of the year. Everything below comes from the Thai Meteorological Department (TMD), the Department of National Parks and other Thai government sources, checked on 19 September 2026.

The short answer

Region	Covers	Drier months	Wettest month	Also watch
Andaman coast	Phuket, Krabi, Koh Phi Phi, Koh Lanta	December to March	September, 429 mm	Some marine parks close between May and October
Gulf islands	Koh Samui, Koh Phangan, Koh Tao	February to April	November, 361 mm	Ang Thong marine park shut 1 November to 15 December 2026
East	Pattaya	November to February	September, 352 mm
Bangkok		November to March	September, 326 mm	PM2.5 haze from late in the year into the next
North	Chiang Mai	November to March	August, 238 mm	Smoke haze, roughly November to April

The months are read from TMD’s 1991 to 2020 averages, set out in full further down. Hua Hin does not fit neatly into any row, and has its own section.

Three seasons, officially

TMD divides the year into three seasons:

• Hot: mid-February to mid-May.
• Rainy: mid-May to mid-October.
• Cool: mid-October to mid-February.

It adds that the rains can start a week or two early or late, and each year it declares the actual dates. The 2025 to 2026 cool season began on 23 October 2025, the hot season on 22 February 2026, and the rainy season on 15 May 2026. When it declared the rains, TMD said upper Thailand’s would last to about mid-October, and the south’s east coast would keep raining until December. The 2026 to 2027 cool season had not been declared when we checked.

TMD calls April the hottest month of the year, especially in upper Thailand, and September the month with the heaviest rainfall for most of the country. The Tourism Authority describes November to February as the cooler months, with temperatures of 18 to 28°C.

Two monsoons, two coasts

This is the part that catches people out. Thailand’s rain comes from two monsoons blowing from opposite directions, and each soaks a different coast.

The south-west monsoon, mid-May to mid-October, blows in from the Indian Ocean. TMD says it brings more rain to coasts and windward mountain slopes than anywhere else. The coast facing it is the Andaman, which TMD’s climate regions call the southern west coast: Ranong, Phang Nga, Phuket, Krabi, Trang and Satun.

The north-east monsoon, mid-October to mid-February, brings cold, dry air to the north and north-east. On its way south it picks up moisture from the Gulf of Thailand, so TMD says it brings heavy rain to the south’s east coast, which can see floods. That is TMD’s southern east coast region: ten provinces including Prachuap Khiri Khan and Surat Thani, down to Narathiwat.

So when Phuket is at its wettest, in September, Koh Samui is having a moderate month, and by the time Samui’s rains peak in November, the Andaman side’s are falling away.

Rain month by month

TMD’s normal rainfall for each region, in millimetres, averaged over 1991 to 2020:

Month	Andaman coast	Gulf coast (south)	Bangkok	East (incl. Pattaya)	North (incl. Chiang Mai)
January	50.2	112.0	21.8	22.9	10.6
February	30.2	44.2	17.7	28.4	9.8
March	103.6	89.8	48.6	66.5	32.4
April	162.2	85.9	95.0	102.0	70.5
May	300.6	132.6	207.1	205.3	173.3
June	335.5	118.7	195.8	259.1	153.3
July	349.4	119.6	183.4	289.0	189.6
August	419.2	130.6	211.7	285.6	237.8
September	429.2	147.0	325.8	351.7	222.0
October	368.8	253.6	244.2	218.6	117.0
November	193.1	361.4	43.5	48.2	26.9
December	91.5	296.0	12.1	9.8	10.6

Read these as regions, not towns. Each column averages several provinces. The Gulf coast column covers ten provinces, from Prachuap Khiri Khan down to Narathiwat on the Malaysian border, and the east column covers eight, including Chon Buri, where Pattaya is. TMD’s figures for individual weather stations could not be read, so no town-level number is given here.

Coast by coast

The Andaman coast: Phuket, Krabi, Koh Phi Phi, Koh Lanta

The driest stretch is January and February, with December and March not far behind. Rain climbs through April, passes 300 mm in May, and peaks in September. When it declared the 2026 rains, TMD warned of waves of 2 to 3 metres in the Andaman Sea, and more than 3 metres in thunderstorms, and advised small boats to stay ashore on those days. Several ferries on this coast run only part of the year, which our island ferry guide covers operator by operator.

Marine parks close in the wet months. The Department of National Parks’ 2026 closure announcement sets these dates for the Andaman parks:

Park	What closes	2026 dates
Mu Ko Surin (Phang Nga)	The whole park	16 May to 14 October
Mu Ko Similan (Phang Nga)	Ko Similan (island 8) and Ko Miang (island 4)	16 May to 14 October
Laem Son (Ranong and Phang Nga)	Ko Kam Tok, Ko Yipun, Ko Khang Khao	16 May to 14 October
Mu Ko Lanta (Krabi)	Ko Rok, with its bungalows and campsite	1 June to 30 September
Mu Ko Lanta (Krabi)	Ko Ha, Hin Muang, Hin Daeng	1 August to 30 September
Hat Noppharat Thara-Mu Ko Phi Phi (Krabi)	Maya Bay, Loh Samah, Ao Loh Ko	1 August to 30 September
Ao Phang Nga	Ko Lawa Yai, Ao Khian, Ko Boi Yai, the bungalow zone and campsite	1 August to 30 September

The same announcement closes several Similan sites and Phi Phi’s Ko Yung indefinitely, and lists no closure for Sirinat National Park in Phuket. Further south, it closes islands in Tarutao and Mu Ko Phetra from 1 June to 30 September, and Ko Muk and Ko Kradan in Trang from 1 August to 30 September.

The Gulf islands: Koh Samui, Koh Phangan, Koh Tao

Here the pattern runs the other way. June to September is fairly even at around 120 to 150 mm a month. TMD says that in the second half of October a low-pressure trough moves down over the south and the Gulf, and that in November the south keeps getting abundant rain, especially on the east coast, while the rest of the country turns dry. November is the wettest month, and December is not far behind. TMD says rain in the south falls off in January, especially on the east coast, and February to April is the driest stretch.

The UK government’s advice puts the rainy season for Koh Samui at November to March. TMD’s averages show the heaviest rain earlier, from October to December.

Park closures. Mu Ko Ang Thong National Park (on our Koh Samui page) is closed entirely from 1 November to 15 December 2026, and so are the islands of Mu Ko Chumphon. In Than Sadet-Ko Pha-ngan National Park, the Khao Ra hiking trail is closed from 15 May to 15 November 2026.

Hua Hin

Hua Hin sits in TMD’s southern east coast region, but that region stretches ten provinces south to the Malaysian border, and TMD publishes no Hua Hin figure we could read. Hua Hin Municipality, in a 2023 briefing document to a Senate budget committee, says the town lies in the rain shadow of the Tenasserim hills, so it gets little rain in the main rainy season, and that most of its rain falls in October and November.

In Khao Sam Roi Yot National Park, south of the town, the 2026 announcement closed the Khao Daeng viewpoint and the Tham Sai and Tham Kaeo caves from 1 to 30 June.

Pattaya and the east

Pattaya’s province, Chon Buri, is in TMD’s eastern region. Its rain builds from May, stays heavy from June to August and peaks in September, then drops away sharply: the eastern average for December is under 10 mm.

Bangkok

Bangkok’s rain peaks in September and October, and December to February are very dry. In April, the hottest month, the average daily maximum is 35.6°C.

Haze. When the Cabinet approved its 2026 measures on forest fires and haze, the government said fine-dust (PM2.5) levels in Bangkok and its surroundings rise every year from late in the year into the start of the next. The UK government’s advice says Bangkok and Chiang Mai are affected by seasonal smoke haze from farmers burning trees and crops.

Chiang Mai and the north

From December to February the north’s average daily minimum is 16 to 17°C, which TMD’s own scale classes as cool (16.0 to 22.9°C) rather than cold. April is the hottest month, with an average daily maximum of 37.2°C. The rains build from May and peak in August and September, and November to March is dry.

The smoke season. The Government Public Relations Department says the north is usually the region worst affected by PM2.5, especially in the dry season, roughly November to April. On 5 April 2026, Chiang Mai, Lamphun and Phayao were declared emergency disaster-assistance zones for forest fires, haze and PM2.5. The criteria for that declaration include PM2.5 above 125 micrograms per cubic metre, averaged over 24 hours, for five days running. The UK government’s advice names Chiang Mai too.

The green season. The Tourism Authority calls mid-May to mid-October the north’s green season.

Trails close in the rains. In Doi Inthanon National Park, the Kew Mae Pan nature trail is closed from 1 June to 31 October 2026. In Doi Suthep-Pui National Park, the Doi Pui summit and campsite are closed from 1 May to 30 September 2026.

High season and low season

We found no official Tourism Authority definition of high and low season. The closest it comes is calling October to December 2025 “Thailand’s high season” when announcing its year-end festivals, and, in July 2026, describing the rainy season as “a shoulder travel period” rather than an off-period.

In the water

The UK government’s advice warns that people have drowned in strong riptides, especially in the monsoon season, and to obey red flags on beaches. On the Andaman side that means the south-west monsoon, roughly May to October. For the Gulf, TMD’s outlook for the 2025 to 2026 cool season forecast waves of 2 to 4 metres from late October to early January. That was a forecast for one season, not an average, but it shows when the Gulf’s rough weather comes.