When does Thailand's rainy season start?

On average, rain jumps clearly from May in almost every region (Bangkok goes 84 → 179 mm from April to May). The official start date of any given year is declared by the Thai Meteorological Department.

Which month is Thailand's wettest?

For most of the mainland it's September (Bangkok 297 mm); for the north it's August (Chiang Mai 412 mm); for the lower Gulf it's November (Songkhla 495 mm — that month's highest figure in the whole table).

Can I do a beach trip in the rainy season?

Yes, if you pick the right coast: in the middle of the year the upper Gulf and Hua Hin run clearly lighter than the Andaman (July: Hua Hin 146 vs Phuket 178 vs Trat 588 mm), and the lower Gulf around Samui is also lighter than the Andaman in most mid-year months.

When is Samui's rainy season?

The opposite of the mainland — heaviest in October–December (Surat Thani measuring point: 316 / 300 / 196 mm), while the middle of the year is comparatively light (June 154, August 161 mm).

Thailand Weather dataSep 12, 20268 min read

When Is Thailand's Rainy Season, Really? 12 Cities, 10 Years of Rain Data

Motorbike rider under an umbrella in heavy rain at a Bangkok night market

Ask when Thailand's rainy season is and the standard answer is May to October. That's true for the north, northeast and central plains. But head to Samui or Hat Yai and the numbers flip: Songkhla averages 495 mm in November — nearly double Bangkok's wettest month — right when Bangkok residents are reaching for jackets.

This page uses ten-year monthly rainfall averages (2016–2025) across 12 cities, the same dataset behind the weather calendar in rooozt's search bar, to answer the question region by region, in numbers — not with one answer for the whole country.

Why Thailand's rain runs on more than one clock

The upper half of the country takes its rain from the southwest monsoon, roughly May to October. The lower Gulf coast gets its heaviest rain from the northeast monsoon instead, from late in the year into the next. The Andaman coast faces the southwest monsoon head-on, so its rain arrives hard from May and runs long into the year's end.

The official start and end dates of each year's rainy season are declared by the Thai Meteorological Department (link at the end) — our numbers describe the average SHAPE of each coast's rain, not this year's declaration.

Lush green rice terraces at Mae Chaem near Chiang Mai under rain clouds — Mae Chaem rice terraces near Chiang Mai in the green season — the north takes the southwest monsoon head-on, peaking in August (Chiang Mai 412 mm)

Region by region: whose rain rises when

The full 12-city × 12-month table is at the end of the article — this is what it says when you read it one region at a time.

North / northeast — one clear peak in Aug–Sep (Chiang Mai 412 mm in August · Khon Kaen 295 mm in September), then a very fast dry-down after October: Nov–Feb Chiang Mai falls from 68 to 9 mm.
Central / east — a steady climb to a September peak (Bangkok 297 · Pattaya 242 mm), except the mountain-backed east coast: Trat (Ko Chang) is the whole table's rain champion at 588 mm in July.
Andaman — the season opens in May (Phuket 230 · Krabi 250 mm) and runs to an October peak (290/296 mm) — the "Andaman rainy season" is longer than mainlanders think.
Lower Gulf — the contrarian coast: Songkhla 495 mm in November · Surat Thani (Samui) 316/300 mm in Oct–Nov, while Songkhla's July is just 131 mm — the mid-year months everyone avoids as "rainy season" are this coast's lightest.
Hua Hin / Prachuap — the rain shadow: no month all year reaches 250 mm, peaking at just 232 mm (October). The beach answer for the middle of the rainy season.

Ko Samui beach with coconut palms under thick monsoon clouds — Monsoon clouds over a Ko Samui beach — the lower Gulf gets its heaviest rain Oct–Dec, opposite the mainland (Surat Thani point: 316/300/196 mm)

The practical version: what to avoid this month, and where to go instead

May–August: avoid the Andaman and the mountain-backed east coast → the upper Gulf (Hua Hin, Pattaya) and lower Gulf run clearly lighter (July: Hua Hin 146 vs Phuket 178 vs Trat 588 mm). September is the mainland's most broadly wet month; if you must travel, Hua Hin has that month's lowest figure (165 mm).

October is the transition — the north starts drying (Chiang Mai down to 178 mm) while the Andaman is still peaking. November–December the whole country dries out EXCEPT the lower Gulf, which is just entering its own peak (Songkhla 495 → 375 mm). City-level detail is in the master weather page and the Phuket monthly page linked below.

What this dataset cannot tell you

Use the table to pick your month and coast; use a real forecast to pick your days.

Mean monthly rainfall doesn't say how many days it rains or at what time — a one-hour afternoon storm and all-day drizzle can produce the same mm.
A 10-year mean is not your year, especially in El Niño/La Niña years.
The official rainy-season start date is declared by the Thai Meteorological Department each year.
Measuring points are a ~9 km grid — small islands use the nearest mainland value (Samui uses the Surat Thani point, Ko Chang uses the Trat point).

Monthly rainfall, 12 cities (mm/month, 10-year mean) — grouped by region

Same thresholds as rooozt's weather calendar: under 50 mm = low rain, over 150 mm = heavy rain — the table scrolls sideways on mobile.

City	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Chiang Mai	20	26	9	58	206	240	364	412	336	178	68	25
Chiang Rai	37	23	15	66	190	253	305	354	287	176	82	29
Khon Kaen	8	17	67	61	176	137	220	198	295	169	40	6
Bangkok	23	35	45	84	179	152	207	193	297	245	82	19
Kanchanaburi	19	31	51	69	170	109	145	153	257	279	85	15
Pattaya/Chonburi	19	44	68	96	166	99	176	146	242	232	72	17
Hua Hin/Prachuap	63	42	49	77	146	106	146	126	165	232	129	37
Trat (Ko Chang)	49	56	127	156	301	401	588	478	529	319	114	30
Phuket	78	34	62	116	230	195	178	207	241	290	246	140
Krabi	84	52	77	154	250	192	202	218	251	296	274	154
Surat Thani (Samui)	125	40	53	149	204	154	185	161	237	316	300	196
Songkhla/Hat Yai	149	64	54	127	189	142	131	135	169	285	495	375

Low rain< 50 mmMiddle50–150 mmHeavy rain> 150 mm

Values at each province's city-centre coordinate — Samui and Ko Chang use mainland measuring points (Surat Thani / Trat), so on-island figures may differ. Single years differ a lot from the mean; always check a real forecast near your travel date.

Official links to check for yourself

Thai Meteorological Department ↗ (opens in a new tab)
The real forecast and each year's official rainy-season declarations — our table is an average, not a forecast

Frequently asked questions

When does Thailand's rainy season start?: On average, rain jumps clearly from May in almost every region (Bangkok goes 84 → 179 mm from April to May). The official start date of any given year is declared by the Thai Meteorological Department.
Which month is Thailand's wettest?: For most of the mainland it's September (Bangkok 297 mm); for the north it's August (Chiang Mai 412 mm); for the lower Gulf it's November (Songkhla 495 mm — that month's highest figure in the whole table).
Can I do a beach trip in the rainy season?: Yes, if you pick the right coast: in the middle of the year the upper Gulf and Hua Hin run clearly lighter than the Andaman (July: Hua Hin 146 vs Phuket 178 vs Trat 588 mm), and the lower Gulf around Samui is also lighter than the Andaman in most mid-year months.
When is Samui's rainy season?: The opposite of the mainland — heaviest in October–December (Surat Thani measuring point: 316 / 300 / 196 mm), while the middle of the year is comparatively light (June 154, August 161 mm).

Where the numbers come from: Every rainfall figure in this article is a 2016–2025 monthly mean from the ERA5 dataset (via the Open-Meteo Historical Weather API) at each province's coordinate — a 10-year statistic, not a forecast for your trip. Any single year can differ a lot from the mean. This is the same dataset the weather calendar in rooozt's search bar displays.