The short answer: November to April. The north-east monsoon brings dry, settled weather and calm seas to this side of Thailand, all the ferry routes operate and every marine park is open. December to February is the most comfortable and the busiest; March and April are hotter, quieter and have the clearest water.
From about May to October the south-west monsoon blows straight onto the coast. It does not rain all day — there are often long sunny spells — but the sea is rougher, some routes stop and several parks close.
Month by month
| Months | Weather & sea | Open / closed | Crowds & prices |
|---|---|---|---|
| November | Rain easing, seas calming; some showers early in the month | Similan and Surin open; island-hopping routes restart | Moderate, rising |
| December – February | Dry, sunny, a little cooler; calm sea | Everything open | Peak — book ahead, highest prices |
| March – April | Hot and dry; calmest sea and best visibility | Everything open; Songkran (Thai New Year) mid-April | Moderate; good value after Easter |
| May – June | Monsoon arrives; showers and swell, sunny breaks | Similan and Surin close mid-May; long speedboat routes stop | Low; big discounts |
| July – August | Mixed: wet spells, sunny days, rough west-facing beaches | Maya Bay typically closes in August; Lanta and small islands very quiet | Low, with a bump for school holidays |
| September – October | Wettest months; frequent boat cancellations | Maya Bay reopens around October; Similan and Surin reopen mid-October | Lowest |
The dry season (November to April)
This is when the Andaman coast looks like its photographs. Days are sunny, humidity is lower and the sea lies flat enough for long speedboat crossings. Daytime temperatures are typically around 30–33 °C, climbing a few degrees in March and April.
The cost is company. From mid-December to the end of February, and again at Chinese New Year, popular hotels fill, Maya Bay time slots sell out and Similan quotas are reached. Book accommodation, the Similan Islands and early-bird Phi Phi trips well ahead. March and April are our favourite months: hot, yes, but quieter, cheaper and with the best underwater visibility — and the main whale-shark window at Richelieu Rock.
The green season (May to October)
The south-west monsoon brings rain in bursts, strong onshore wind and a persistent swell on west-facing beaches. What that means in practice:
- Closed: the Similan and Surin Islands (about mid-May to mid-October), Koh Rok and Koh Tarutao, and Maya Bay for around two months (typically August and September).
- Suspended: most long speedboat routes — Lanta to the Trang islands and Lipe, and Lipe to Langkawi. Many small-island resorts on Kradan, Ngai, Phayam and parts of Lanta shut.
- Still running: Phuket–Phi Phi–Krabi ferries, Phang Nga Bay tours (the bay is sheltered), Krabi’s inland trips and mangrove kayaking, and Pak Bara–Koh Lipe on a reduced timetable.
- Swimming: red flags fly often on Phuket’s and Khao Lak’s west coasts. Take them seriously. East-facing and sheltered beaches — Railay East, Ao Nang on calm days, Koh Yao, Phuket’s east coast — are safer bets.
In return you get hotel rates that can be half the peak price, green hills, empty viewpoints and dramatic skies. If you come in these months, base yourself in Phuket, Krabi or Koh Yao rather than on a small outer island, keep plans flexible and book boat trips a day or two ahead once you can see the forecast.
Beyond Thailand
- Langkawi has a similar but milder pattern: driest from about December to March, wettest in September and October. The ferry to Koh Lipe runs only in the dry season.
- Mergui Archipelago: the same November-to-April window, when access is possible at all.
- Andaman Islands (India): best from about November to April. They catch both monsoons, with the heaviest rain from roughly May to September and cyclone risk in the surrounding months.
Festivals worth knowing
- Songkran (13–15 April): Thai New Year and a nationwide water fight, liveliest in Patong and Phuket Town.
- Phuket Vegetarian Festival (usually October): nine days of processions, firecrackers and remarkable street food in Phuket Town. Not for the squeamish.
- Loy Krathong (November full moon): candlelit floats launched on the water on beaches and lagoons across the coast.
- Laanta Lanta Festival (around March): music and food in Lanta Old Town.