Thailand’s two island coasts run opposite rainy seasons. A month-by-month guide to choosing Phuket, Krabi, and Phi Phi versus Samui, Phangan, and Tao for your dates.
Thailand has two island coasts, and they do not share a weather calendar. The Andaman Sea side (Phuket, Krabi, Phi Phi, Koh Lanta) and the Gulf of Thailand side (Koh Samui, Koh Phangan, Koh Tao) run on roughly opposite rainy seasons. Choosing the wrong coast for your dates is one of the most common — and most avoidable — Thailand planning mistakes.
This guide explains the difference, when each coast is at its best, and how to choose when your dates are fixed.
The one-sentence version
Andaman coast: driest and calmest roughly November to April. Gulf coast: driest and calmest roughly January to August, with its wettest stretch around October to December. The months overlap generously, so most of the year at least one coast is in good shape.
These are broad patterns, not promises. Weather varies year to year, so treat the ranges below as planning guidance and check current forecasts before booking boats or beach time.
Why the two coasts differ
The Andaman side faces the open Indian Ocean and takes the full force of the southwest monsoon, which brings heavier seas and frequent rain roughly May to October. Some smaller islands and boat routes reduce service or close entirely in this period.
The Gulf side is more sheltered. Its islands sit behind the Malay Peninsula, so the southwest monsoon loses much of its strength before reaching them. The Gulf's heavier rain arrives later in the year, driven by the northeast monsoon, peaking around October to December.
Same country, opposite calendars. That is the entire trick.
Andaman coast: Phuket, Krabi, Phi Phi, Koh Lanta
Best window: roughly November to April. Calm seas, clear water, reliable boat trips, and the most predictable beach weather. December and January are also the busiest and most expensive weeks, so book accommodation and popular boat trips early.
Shoulder months: roughly October and May. A mix of sunny days and storms. Prices drop, crowds thin, and a flexible traveler can do very well — but keep boat plans cancellable.
Rainy stretch: roughly May to October. Longtail and speedboat trips are frequently cancelled for sea conditions, swimming can be dangerous at exposed beaches (respect the red flags), and some smaller islands effectively shut down. It is not a write-off — rain often comes in bursts rather than all day — but do not build a once-in-a-lifetime snorkel day on an October Andaman forecast.
Gulf coast: Koh Samui, Koh Phangan, Koh Tao
Best window: roughly January to August. This is the great advantage of the Gulf: it is at its best during the northern-hemisphere summer, exactly when the Andaman side is at its wettest. June, July, and August are genuinely good months on Samui, Phangan, and Tao.
Shoulder month: roughly September. Transitional, with increasing showers toward the end of the month.
Wettest stretch: roughly October to December. November is often the Gulf's heaviest rain month — a mirror image of the Andaman's best season. Full-moon-party travelers heading to Koh Phangan in November should pack for rain and keep expectations flexible.
A month-by-month cheat sheet
- January – March: Both coasts good. Andaman at its absolute peak.
- April: Both coasts workable; hot everywhere, Andaman seas still calm early in the month.
- May: Transition month. Gulf becomes the safer bet as the Andaman monsoon builds.
- June – August: Gulf clearly stronger. Andaman wet and rough.
- September: Gulf still reasonable, fading late in the month; Andaman still rainy.
- October: The trickiest month for islands on both sides. Consider the north or Bangkok instead, or accept flexible plans.
- November: Andaman improving rapidly; Gulf at its wettest.
- December: Andaman excellent; Gulf drying out toward the end of the month.
What if your dates are fixed?
Most travelers cannot choose their vacation month. If yours is locked, the coast decision follows the calendar rather than your preferences:
- Traveling June to September? Build your island time around Samui, Phangan, or Tao — not Phuket or Phi Phi.
- Traveling November to April? The Andaman side delivers the classic limestone-scenery trip at its best.
- Traveling October or May? Either coast is a gamble. Plan a route that works even if beach days shrink — Bangkok, Chiang Mai, and the north are season-proof anchors, and our best time to visit Thailand guide covers the regional picture month by month.
Don't forget the transfer math
The coast decision also changes your logistics. Gulf islands are usually reached via flights to Samui or Surat Thani plus a ferry; the Andaman side is served by Phuket and Krabi airports with speedboats and ferries onward. Ferries are weather-dependent on both coasts, and in the wetter months they are the first thing to be cancelled.
Whatever coast you choose, leave a generous buffer between your last ferry and any flight — ideally a night on the mainland or near the airport. We cover the details in why AI itineraries break in Thailand, and the Phuket vs Krabi vs Koh Samui comparison helps once you have picked a coast.
The calm way to plan it
This is exactly the kind of decision Sabai Journey is built for. Tell the app your dates and it steers your island time toward the coast in season, builds ferry-to-flight buffers into your route, and keeps a weather-flexible backup for boat days — so a monsoon forecast reshuffles your plan instead of ruining it.
Gulf vs Andaman FAQ
Can I visit both coasts in one trip?
Yes, if you have two weeks or more — but they shine in different months, so one coast will usually be better than the other during your dates. In a shorter trip, pick the in-season coast and go deeper rather than splitting a week between two islands and two sets of transfers.
Is the Gulf coast less beautiful?
Different, not less. The Andaman has the dramatic limestone cliffs; the Gulf has flatter, gentler island scenery and some of the country's best diving around Koh Tao. Many travelers end up preferring whichever coast they caught in good weather.
What about October — is anywhere good?
October is the shoulder of the shoulders for islands. Bangkok, Chiang Mai, and the northern mountains are far more reliable, and by late October the Andaman side is often turning the corner. If your heart is set on an October island, keep plans flexible and bookings cancellable.
Do these patterns hold every year?
No. Monsoons arrive early some years and late in others, and climate variability is real. Use these ranges to choose a coast, then check actual forecasts in the weeks before you travel — especially before committing to boat trips.