Thailand has an Andaman west coast (Phuket, Krabi, Phi Phi, Khao Lak, Similans) and a Gulf east/south coast (Samui, Phangan, Tao, Hua Hin, the Trat islands). They do not share a weather mood. The southwest monsoon soaks the Andaman roughly May–October. The northeast monsoon soaks many Gulf-facing beaches roughly October–December. Book the wet side by accident and you will spend the week in a lobby, staring at a flag that says no boats.
Two seas, two clocks
November–April is the classic Andaman window: drier, calmer, the season Phuket and Krabi sell. May–October is when the Gulf islands are often the better beach bet while the Andaman takes swell. This is climate, not a guarantee. A “dry” month can still dump rain at 16:00. A “wet” month can still give you three bright mornings and one cancelled longtail.
The Similan gate is the sharpest Andaman fact we will give you: Mu Ko Similan closed about 16 May–14 October 2026, open about 15 October–15 May. Park fee for a foreigner adult is commonly cited around 500 THB; daily cap cited 3,850. That is not a “maybe the operator knows a way”. See Park permits and Snorkel or dive.
Mainland Andaman — Krabi town, Ao Nang, Khao Lak — still exists in the wet months. You get karst and jungle and afternoon storms. You do not get a reliable Phi Phi snorkel every day. Gulf Samui has an airport; Tao is a dive-course island; Phangan is a party reputation with quiet corners if you leave the full-moon beach.
Pack for spray even in a good month. A roll-top dry bag is the difference between a wet phone and a boring, working phone. Reef-safe claims on sunscreen are marketing more often than chemistry; a rash vest does the honest job.
The monsoon mechanics — southwest versus northeast, why the Gulf is the exception — are on Monsoon seasons. Gateway’s month-by-month planning voice is When to visit. This page is only the fork: which sea, which month.
Twelve-month suitability (beach / boat, not a rainfall gauge)
These bars are a planning suitability sketch for a first-time beach week — calm-ish seas, a reasonable chance the longtail runs — not millimetres and not a promise. Peak Andaman sits in the cool-season months. Peak Gulf beach time sits in the Andaman’s wet months, then dips when the northeast monsoon hits the Gulf.
Andaman — Phuket / Krabi / Similan season
Gulf — Samui / Phangan / Tao
| Month | Andaman in one line | Gulf in one line |
|---|---|---|
| Jan–Feb | Best first-timer window | Good, rain usually easing |
| Mar–Apr | Hot, still boatable; April is fierce inland too | Hot; Songkran 13–15 April 2026 is water regardless of sea |
| May–Sep | Monsoon, Similans shut from ~16 May | Usually the beach you wanted |
| Oct | Andaman recovering; Similans from ~15 Oct | Often the month to avoid for Gulf sand |
| Nov–Dec | Andaman opening into prime | Nov can still be the Gulf’s messy month |
Islands versus mainland
Krabi and Khao Lak give you Andaman scenery without committing to a longtail every morning. You can still lose a day to wind. Phuket is an airport with beaches attached; west-coast swell in the wet season is why people look confused in July.
Samui has an airport and a road. Tao is why Open Water is a Gulf product in the Andaman’s rain. Phangan is not only a full moon; it is also a west-coast sunset if you leave the party beach. Trat / Ko Chang is a different Gulf clock again — closer to Cambodia, not a Samui clone — check local boat notices rather than copying the Samui bar chart as law.
Hua Hin and Cha-am are Gulf mainland: easier from Bangkok, less “limestone postcard”, still the same sea. They are a weekend, not a substitute for Krabi.
Crossing both coasts in ten days is a travel video. The minivan hours will eat the swimming hours. Pick one. If you have three weeks, you can still pick one and go north instead of collecting piers.
Rain does not close the country. It closes boats and Similan. Inland parks — Khao Yai, Doi Inthanon — run on yet another feel: afternoon storms, muddy trails, 2,565 m of cold air on Inthanon. Those are Park permits problems, not coast problems.
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Andaman coast, Gulf coast, Best beaches, Monsoon, Snorkel or dive.