Thailand monsoon season — complete month-by-month breakdown
A detailed guide to Thailand's rainy season: when it starts, how heavy it gets, which regions are wettest, and how it affects tourism and daily life.
When does monsoon start in Thailand?
The southwest monsoon typically arrives in Thailand in mid to late May. The onset is not a single dramatic event but a gradual increase in afternoon thunderstorm frequency over 1-2 weeks. The southern Andaman coast sees rain first (mid-May), followed by the central plains and Bangkok (late May to early June), and finally the northeast Isan region (June). The monsoon retreats in reverse order, typically clearing from the north in October and the south by November-December.
May — monsoon onset
The transition from hot to wet. The first significant rains arrive in mid-May, bringing relief from the April heat. Temperatures drop slightly (33-35°C to 31-33°C) but humidity jumps. Rain is initially sporadic — perhaps 3-4 days per week with afternoon thunderstorms. The countryside begins turning green. Tourism starts dropping off as beach weather on the Andaman coast deteriorates.
June and July — building momentum
Rain frequency increases to 4-5 days per week. The pattern is remarkably consistent: clear or partly cloudy mornings, cloud buildup from noon, thunderstorms 2-5 PM, clearing by evening. Bangkok can see dramatic flooding during heavy afternoon storms as drainage struggles. The Gulf coast (Koh Samui, Koh Phangan) is actually in a relatively dry window — this is peak party season on the Gulf islands.
August and September — peak wet season
The wettest months for most of Thailand. September is typically the peak — Bangkok averages 330 mm, and the central plains can see widespread flooding. Continuous rain (not just afternoon storms) can occur when monsoon depressions pass through. Rivers and reservoirs fill. Rice paddies are at their most photogenic — vivid green fields stretching to the horizon. Tourism is at its lowest and prices are at their cheapest.
October — monsoon retreat and Gulf coast wet season
The monsoon begins retreating from northern Thailand in early October. The Andaman coast starts clearing. However, October is actually the START of the Gulf of Thailand's wettest period. Koh Samui, Koh Phangan and the Gulf coast see their heaviest rain from October to December as the retreating monsoon pushes moisture eastward. This is why the two coasts have opposite peak seasons.
Practical tips for monsoon travel
Monsoon season Thailand is underrated for budget travellers. Prices are 30-50% lower than peak season. The secret: mornings are usually sunny. Plan outdoor activities before noon. Carry a compact umbrella and waterproof phone case. Book the Gulf coast islands (Koh Samui, Koh Phangan, Koh Tao) from May to September — they are in their dry window while the Andaman coast is wet. Take advantage of monsoon-season hotel promotions at luxury resorts.
Frequently Asked Questions
Does it rain all day during Thai monsoon? ▾
No — the typical pattern is clear mornings, cloud buildup at midday, thunderstorm from 2-5 PM and clearing by evening. All-day rain only occurs during monsoon depression events, which happen a few times per season.
Can I still enjoy a beach holiday during monsoon? ▾
Yes — the Gulf coast (Koh Samui, Koh Phangan, Koh Tao) is driest from May to August, which is peak monsoon on the Andaman coast. Plan your beach time on the Gulf side during monsoon months.
Is Bangkok safe during monsoon flooding? ▾
Most tourists are not seriously affected by Bangkok flooding — it tends to affect suburban low-lying areas more than the central tourist districts. However, the 2011 floods were an exception. Check river levels during September-October if planning a trip.