TL;DR: Thailand’s rainy (green/monsoon) season runs roughly May through October across the mainland, the north, and the Andaman coast, driven by the southwest monsoon, with September-October the wettest stretch there. The Gulf coast (Koh Samui, Koh Phangan, Koh Tao) is different, it’s sheltered from that system and instead catches the northeast monsoon from roughly October through December, with November its wettest month by far. In practice, rain is usually a heavy but short afternoon or evening downpour, not all-day rain, and it’s the cheapest, quietest, greenest time to travel almost everywhere it applies, traded against rougher seas, some ferry and resort closures, and Bangkok’s flood-prone patch. All prices ฿33 = US$1 (July 2026).
Most “best time to visit Thailand” content mentions rainy season in a single line and moves on. That undersells it, and skips what travellers actually need to know: what the rain feels like day to day, how differently it plays out on the Andaman coast versus the Gulf islands versus the north, which islands genuinely shut down, and whether it’s worth booking around at all. It often is. This guide goes deep on the season itself, a companion to outthailand.com’s best time to visit Thailand overview, which covers the full year and every season side by side.
Prices are in Thai baht (THB) with US dollars in parentheses at ฿33 = US$1 (July 2026), given as ranges since discounts vary by property and month.
Table of Contents
- What causes Thailand’s rainy season?
- What is the rain actually like?
- Region by region, month by month
- What closes during rainy season?
- Is rainy season worth visiting?
- How to plan a rainy-season trip
- FAQ
What causes Thailand’s rainy season?
Thailand’s wet season is driven by the Asian monsoon system, and specifically by which monsoon a given coast faces. The southwest monsoon, roughly May through October, pushes moist air off the Indian Ocean into the mainland, the north, and the west-facing Andaman coast (Phuket, Krabi, Koh Lanta, Koh Phi Phi, Koh Lipe). Those regions get progressively wetter from May, peaking around September-October.
The Gulf coast (Koh Samui, Koh Phangan, Koh Tao) sits on the opposite, east-facing side of the peninsula and is largely shielded from that southwest monsoon by the landmass in between. It pays for that shelter with its own wet season instead: the northeast monsoon, roughly October through December, which barely touches the Andaman side at all. That single fact, that Thailand effectively runs two separate rainy seasons on two coasts, is the most useful thing to understand before you book around it.
What is the rain actually like?
Forget the mental image of a grey, rained-out week. The dominant pattern, especially on the mainland, in the north, and on the Andaman coast, is a clear or partly cloudy morning followed by an intense downpour in the afternoon or evening, often lasting somewhere between 20 minutes and two hours, and then clearing. Streets steam, temperatures drop a few degrees, and life carries on once it passes. Multi-day grey, wet spells do happen, most often right at the peak of each region’s wettest month, but they are the exception rather than the everyday reality.
What does change constantly is humidity, which stays high throughout the season regardless of whether it’s actively raining, and sea conditions on exposed coasts, which is where most of the season’s real disruption lives (more on that below), not the rain itself.
Region by region, month by month
This is the detail most generic guides skip: the same calendar month can be a great time on one coast and the worst possible pick on another.
| Month | Andaman coast (Phuket, Krabi, Koh Lanta) | Gulf coast (Koh Samui, Koh Phangan, Koh Tao) | North (Chiang Mai) | Bangkok |
|---|---|---|---|---|
| May | Monsoon starts fast, ~295mm in Phuket | Still mostly dry, sheltered from this monsoon | Rain increasing, ~155mm, air improving | Rainy season begins mid-month |
| June | Rougher seas, regular afternoon downpours | One of the better months, still fairly dry | ~115mm, lush and green, solid month | Frequent short downpours |
| July | Steady monsoon rain and swell continues | Secondary dry window opens, ~100-115mm | ~155mm, wetter but workable | Heavy afternoon rain typical |
| August | Rain persistent, red flags common on west coasts | Still in the secondary dry window | ~225mm, the north’s wettest month | Among the wetter months |
| September | Very wet, approaching the Sept-Oct peak | Dry window closing, rain picking up | ~200mm, still wet but easing | Wettest month, ~335mm over ~21 days |
| October | Wettest tail, ~315-325mm in Phuket; some island closures still in effect | Own wet season starting, ~295mm in Koh Samui | Rain easing toward the dry season | Flood risk continues, easing late month |
| November | Monsoon easing, seas calming, dry season nearing | Wettest month by far, ~445mm in Koh Samui | Dry season begins, clear skies | Dry season begins |
Rainfall figures are long-term monthly averages compiled from Climates to Travel and outthailand.com’s regional best-time guides; individual years vary. See Sources.
What closes during rainy season?
This is where the season bites hardest, and it’s genuinely regional rather than a blanket “everything shuts” situation. Koh Lanta is the clearest example: around 80% of the island’s businesses, restaurants and bars close for stretches of low season, the island’s main coworking space, KoHub, shuts completely from May 1 to October 31, and the national marine park covering Hin Daeng, Hin Muang, Koh Haa and Koh Rok closes to divers and snorkellers from roughly mid-May to mid-October, a full closure of those specific dive sites, not just rough-weather cancellations. Several seasonal boat routes to Phuket, Phi Phi, Ao Nang and Koh Lipe stop running entirely, though the overland minivan-and-car-ferry route from Krabi Airport keeps running year-round as a reliable fallback. See outthailand.com’s best time to visit Phuket guide for how the wider Andaman coast handles the same stretch, since not every island closes as completely as Koh Lanta.
Elsewhere the picture is milder: Phuket and Krabi stay open with reduced services and rougher seas rather than wholesale shutdowns, Bangkok and Chiang Mai operate normally year-round, and the Gulf islands (Koh Samui, Koh Phangan, Koh Tao) barely scale back at all during the mainland’s rainy season, since it isn’t their wet season. Whatever the region, expect red no-swimming flags on exposed beaches during their respective wet peaks, and build slack into ferry-dependent island-hopping plans.
Is rainy season worth visiting?
For a lot of travellers, yes, deliberately. The trade you’re making is real but so is the upside. Prices drop and crowds thin out almost everywhere the season applies, often the single biggest lever if budget or breathing room matters more than guaranteed sunshine. Landscapes hit their best: waterfalls run full, rice paddies turn a deep green, and jungle scenery around Chiang Mai and the islands looks nothing like its dusty dry-season self. And because rain usually means a contained afternoon downpour rather than a lost day, most itineraries survive it intact.
The honest catches are specific, not vague: rougher, sometimes unswimmable seas and reduced boat schedules on the Andaman coast May-October; real closures on islands like Koh Lanta; Bangkok’s flood-prone September-October patch; and November’s washout on the Gulf coast, which is genuinely the one stretch we’d steer most travellers away from if that’s their only island option. Match the region to the month rather than treating “rainy season” as one uniform verdict, and it stops being a reason to avoid Thailand and becomes a reason to pick your coast correctly.
How to plan a rainy-season trip
- Pick the coast before the date. If your travel window is fixed for June-September, the Gulf coast (Koh Samui, Koh Phangan, Koh Tao) is usually the better call over Phuket or Koh Lanta; flip that logic for October-December.
- Build indoor backup into afternoons. Museums, malls, cooking classes, spas and markets absorb the typical downpour window without derailing the day.
- Book flexible accommodation and ferries where seasonal closures are a possibility, especially around Koh Lanta and other smaller Andaman islands.
- Pack for humidity and short, heavy rain, not constant drizzle: a packable rain jacket, a dry bag for electronics, quick-dry clothes and sandals do most of the work. See outthailand.com’s Thailand packing list for the full checklist.
- Fill a rainy afternoon with what’s actually on. Check outthailand.com’s live Thailand events listings for markets, festivals and indoor community events happening during your dates.
Where to next
For the full national picture across all three seasons, start with outthailand.com’s best time to visit Thailand guide. Heading to the Andaman coast, the best time to visit Phuket guide has the fuller month-by-month rundown; heading to the Gulf islands instead, see best time to visit Koh Samui. Packing for the season is covered in the Thailand packing list, and you can browse what’s actually happening during your trip in the live Thailand events listings.
Sources
- Climates to Travel: Phuket Climate: month-by-month rainfall averages for the Andaman coast.
- Climates to Travel: Ko Samui Climate: month-by-month rainfall averages for the Gulf coast, northeast monsoon timing.
- Climates to Travel: Chiang Mai Climate: northern Thailand rainfall averages.
- outthailand.com Best Time to Visit Thailand: consolidated national and regional rainfall figures.
- outthailand.com Best Time to Visit Phuket: Andaman coast monsoon detail, red-flag swimming conditions.
- outthailand.com Best Time to Visit Koh Samui: Gulf coast northeast monsoon detail.
- outthailand.com Best Time to Visit Koh Lanta: island closure specifics (KoHub coworking, marine park, ferry suspensions), sourced there from KoHub’s own seasonal guide.
- outthailand.com Best Time to Visit Bangkok: Bangkok rainy-season and flood-risk figures.
- Xe.com: USD/THB Currency Converter: exchange rate reference, July 2026.