Sea conditions make or break this trip. Here's exactly when to go, when to hold off, and the exact dates the park shuts for the year.
Book February or March. Those two months bring the driest skies, calmest seas and clearest snorkelling water of the year, and every tour runs on schedule. December, January and April are still very good options. The one stretch to avoid outright is October–November: not only is it the wettest, roughest weather of the year, Ang Thong closes entirely from November 1 to around December 15 for its annual monsoon rehabilitation break.
Koh Samui and Ang Thong sit on the Gulf of Thailand side of the peninsula, which runs on the opposite monsoon clock to Phuket and the Andaman coast. Their wet, rough months are October–December; their dry season is roughly December–April. That's why "best time for Thailand" advice from Andaman-coast guides can be misleading here.
| Month | Weather | Sea Conditions | Ang Thong Verdict |
|---|---|---|---|
| January | Cool, drying out, occasional showers | Calm, light NE winds | ✓ Good — book 2–3 days ahead |
| February | Driest, sunniest month | Calmest of the year | ✓ Best — ideal conditions |
| March | Sunniest skies, ~8 hrs/day | Calm | ✓ Best — peak visibility |
| April | Hottest month begins; Songkran Apr 13–15 | Still calm | ✓ Good — book ahead for Songkran |
| May | Hot; first short-shower peak | Building SW-monsoon swell | ◑ Fair — check the forecast |
| June | Warm, short afternoon showers | Moderate | ◑ Fair — good value, fewer crowds |
| July | Reliable summer beach weather | Generally calm on the Gulf side | ◑ Fair — smart pick over the Andaman coast |
| August | Similar to July, lush and green | Generally calm | ◑ Fair — good value, low crowds |
| September | Cloudiest month, rain increasing | Building | ✕ Weak — rising cancellation risk |
| October | Wet season ramps up hard | Rough | ✕ Avoid — frequent cancellations |
| November | Wettest month of the year | Roughest | ✕ Closed Nov 1 (see below) |
| December | Rain easing, festive peak | Improving through the month | ◑ Good after ~Dec 15 reopening |
Temperatures barely move year-round (roughly 27–32°C by day, sea always 28°C+) — rainfall and sea state are the real variables. Meteorological sources vary on exact rainfall totals by a few hundred millimetres depending on station and reference period, but they all agree on the same pattern: driest in February, wettest in October–November.
Whatever week you land on, there's a good boat trip for it.
If your dates land in the Ang Thong Marine Park closure window or you just want to spread the diving days out, Koh Tao and the Koh Nang Yuan sandbar run some of the clearest snorkelling in the Gulf almost year-round, while a Koh Madsum (Pig Island) and Koh Tan trip from Koh Samui stays calm even in shoulder-season swell. Prefer to set your own schedule around the tides? A private speedboat charter or sea-kayaking day can usually work around all but the roughest weather. Live options below.
The same four windows, but framed for planning: when to book early, when to bargain-hunt, and when to build a backup day.
Driest, calmest and clearest of the year. Highest prices and most crowded weekends of the whole season — book 6–8 weeks ahead, especially around Chinese New Year.
Excellent once the monsoon clears (mid-December on) and again through Songkran. Near-peak conditions at slightly lower prices in early January.
The Gulf side is often calmer here than the Andaman coast during its own SW monsoon — a smart pick for European summer holidays. Short showers, thinner crowds, best value of the year.
The wettest, roughest stretch of the year, with frequent cancellations even before the park's formal closure begins on November 1.
Mu Ko Ang Thong National Marine Park closes annually from November 1 to around December 15, a formal closure set by the Thai government for monsoon safety and environmental rehabilitation — it lets the islands' beaches and reefs recover from a year of visitor traffic before the next high season. It is not simply "bad weather cancellations"; no operator can run the standard island-hopping itinerary during this window, whatever the forecast looks like.
A few things worth knowing before you plan around it:
Swap your boat day for the Koh Tao & Koh Nang Yuan day trip — it runs on a similar schedule and rarely gets cancelled outright, even when Ang Thong is shut.
Weather quality is one input — your travel style matters just as much.
February–March for the postcard combination: glassy water, sunset light and near-guaranteed clear viewpoint photos. A private charter is worth the splurge in this window.
February–April for the safest crossings and best underwater visibility — see our snorkelling guide for what conditions actually look like at each stop.
July–August. Choose Samui over Phuket for this stretch — the Gulf is markedly sunnier and calmer than the Andaman coast during its own monsoon.
May, June or September for the lowest prices, thinnest crowds and greenest scenery, accepting the odd short shower.
January–March for the most reliable dry-and-calm run of the year, with May–July as a cheaper backup that still mostly delivers.
October–November for any boat-based plan — pair a fixed trip in this window with an Andaman-coast leg (Phuket, Krabi) instead, since it's dry there.
More questions? The homepage FAQ covers what's included, family suitability and booking basics.
Compare tour options for your travel window — duration, transport type and cancellation policy all in one place.