Southern Thailand has one major climatic peculiarity: its two coasts run on offset seasons, which makes the region accessible almost year-round provided you choose the right coast at the right time.
The Andaman coast (Phuket, Krabi, Koh Lanta, Koh Phi Phi) is at its best from November to April. The southwest monsoon brings substantial rain from May to October: in June and July, waves can reach 2 to 3 metres on Patong beach, and some beaches fly red flags banning swimming. November marks the return of the dry season: the sea calms progressively, the sky clears. December to March concentrates the best conditions: calm sea, blue sky, underwater visibility of 15 to 30 metres. April and May remain decent — a few late-afternoon thunderstorms, but mornings still allow boat excursions.
The Gulf coast (Koh Samui, Koh Phangan, Koh Tao) works in reverse: the best period runs January to September. The rainy season on the Gulf side starts later (September-October) and peaks in November-December — Koh Samui receives more rain in November and December than in any other month. Koh Tao, further north and more sheltered, is accessible almost year-round for diving: even during the rainy period, the dive sites remain workable most days.
Month by month on the Andaman coast: November marks the start of the dry season, the sea recovers its clarity and boats resume regular rotations to the islands. December-January is the peak of high season — accommodation in Phuket and Railay sells out weeks ahead, book 6-8 weeks before departure at least. February is often cited as the best month: perfect weather, glassy sea, attendance slightly lower than during year-end holidays. March remains very pleasant, the first clouds appearing in late afternoon without disrupting outings. April is the pivot: the first monsoon rains arrive sporadically, but mornings stay fine for boat excursions. From May to October on the Andaman, the sea is rough, several beaches close to swimming and some ferries are suspended — better to switch to the Gulf coast then.
Month by month on the Gulf: January to March offers good conditions, with calm seas and clear skies around Koh Samui. April to August is the high season on the Gulf side, with warm waters and reasonable attendance on Koh Tao, which concentrates divers. September-October sees more frequent rainfall but very attractive prices. November-December remains the period to avoid on the Samui side: heavy rain, sometimes rough seas.
Our take: if you have the choice, aim for February on the Andaman coast and May-June on the Gulf. These two windows offer the best balance of weather, attendance and pricing. Travellers combining both coasts in a single trip can plan a circular itinerary: Andaman coast at the start of the trip (November-December), then crossing to Koh Samui and Koh Tao via Surat Thani for a second week in good conditions.
Read also
- Phuket, the great southern island — Patong and Kata beaches, nightlife and excursions to the Phi Phi Islands from Thailand's largest island.
- Krabi, cliffs and turquoise sea — Railay Beach, diving and kayaking at the heart of the Andaman's karst formations.
- Thailand — complete country guide — Formalities, budget, when to go and regions of Thailand.
- Northern Thailand — the other Thailand — Temples, night markets and trekking in the hills of the Thai north.