Thailand's Islands Compared: Andaman vs Gulf and When Each Has Good Weather

One of the most consistent mistakes I see in Thailand trip planning is picking an island without checking which coast it’s on. Thailand’s two main island groups — the Andaman Sea on the west and the Gulf of Thailand on the east — have completely opposite monsoon patterns. An island that’s perfect in February can have closed ferries and deserted resorts by June, while islands forty minutes away by plane are at their sunniest.

I’ve been caught on the wrong side of this enough times (once arriving in Koh Lanta during the week the last restaurant closed for low season, once booking Koh Phangan in October when it rained every single afternoon) to be genuinely emphatic about it: the coast you choose matters as much as the island itself.

What Is the Difference Between the Andaman Sea and the Gulf of Thailand?

The geography is simple: the Andaman coast runs down Thailand’s western side (Phuket, Krabi, Koh Lanta, Koh Phi Phi, Khao Lak, Koh Lipe). The Gulf of Thailand runs down the eastern side (Koh Samui, Koh Phangan, Koh Tao, Koh Chang, Koh Kood). The Malay Peninsula divides them.

The monsoons hit each coast differently. The southwest monsoon, which brings the main rainy season to most of Southeast Asia from May through October, strikes the Andaman coast first and hardest — Phuket and Krabi get sustained rain and rough seas from June through September. Meanwhile, the Gulf islands on the opposite side of the peninsula are often shielded from the worst of it and remain relatively dry through the same months.

Then the pattern reverses. The northeast monsoon arrives from October through January, bringing rain and rough water to the Gulf side while the Andaman coast enjoys its best weather of the year.

The result is that Thailand always has good island weather somewhere — you just have to know where to look for your travel dates.

When Should You Visit the Andaman Coast?

Best months: November through April. This is when the Andaman Coast is at its most spectacular. Seas are calm, visibility for diving runs to 20–30 meters, the water is gin-clear, and the longboats can reach islands like Koh Phi Phi and the Phi Phi Leh lagoons without the swell that closes some routes in the wet season.

Shoulder: May and October. May is the final dry month before the monsoon arrives in earnest — you’ll get some rain but also uncrowded beaches and noticeably cheaper rates. October sees the first improvement after the wet season; the water hasn’t fully settled but conditions are improving and the resort infrastructure is reopening.

Avoid for beaches: June through September. The Andaman is under the full force of the southwest monsoon during these months. Phuket and Krabi are still functional cities — restaurants open, hotels operating — but rough seas mean some boat tours cancel, Koh Phi Phi ferry services can be sporadic, and the islands feel like low-season outposts because they effectively are. If you’re going for the experience of the destination rather than the beach itself, it’s manageable. If you’re going for crystal water and full-day boat trips, wait.

The Andaman islands in brief:

Phuket — the biggest, most developed, and most accessible. Kata and Karon beaches are the better choices for beach quality; Patong is lively nightlife. Old Phuket Town has genuinely excellent food and architecture if you make time for it.

Krabi — limestone karst landscape, four-islands boat trips, and Railay Beach (accessible only by longtail, one of Thailand’s most scenic spots). The base for accessing the best of the Andaman.

Koh Lanta — quieter than Phuket and Krabi, longer beaches, Old Town on stilts over the sea. The right answer when you want the Andaman without the crowds.

Koh Phi Phi — spectacular scenery, limited vehicle access (the main village is walkable), and Maya Bay operating again with regulated entry. Peak-season crowds are real; go in November or early December for the balance.

Khao Lak — north of Phuket, lower-key, and the departure point for Similan Islands liveaboards. The dive access from here is world-class when the season is open (approximately October through May).

When Should You Visit the Gulf of Thailand?

Best months for Koh Samui and Koh Phangan: February through June and January. The Gulf’s dry season doesn’t perfectly mirror the Andaman — it runs from February through June on most Gulf islands, then again from December through January. This is when you get the iconic Koh Phangan and Koh Samui weather: blue sky, flat seas, reliable sunshine.

Best months for Koh Tao: March through September. Koh Tao, positioned slightly farther out in the Gulf, benefits from the monsoon pattern that keeps it drier than Koh Samui during the October-December northeast monsoon. This makes it better for diving visibility during the months when the dive schools are busiest.

Avoid for Gulf islands: October and November. This is when the northeast monsoon hits the Gulf hardest. Koh Samui airport regularly floods. Ferry schedules become unreliable. The rain isn’t the pleasant afternoon-downpour pattern of the dry-season travel month — it’s sustained and can keep you indoors for full days. Some resorts close in November for refurbishment.

December through January is a split: December tends to be good on the Gulf (post-monsoon, transitioning to dry season), but December into January can bring short periods of rain that catch visitors expecting guaranteed sunshine.

The Gulf islands in brief:

Koh Samui — the most developed Gulf island with direct flights from Bangkok (Bangkok Airways runs them, which is why fares are pricier than most domestic routes). Chaweng Beach is the busy end; Bophut’s Fisherman’s Village is worth the evening. Good base for Ang Thong National Marine Park day trips.

Koh Phangan — famous for the Full Moon Party on Haad Rin (monthly, draws thousands). But the north and east coasts of Phangan are a different island entirely — quiet, beautiful, genuinely peaceful beaches that most Full Moon visitors never find. Worth seeking.

Koh Tao — the cheapest place in the world to do a PADI Open Water course (around ฿9,800/$280 for the 3-day certification). Even if you’re certified, Koh Tao’s dive sites are excellent — Japanese Gardens, Shark Island, and Chumphon Pinnacle for experienced divers. Not a relaxing resort destination; it’s an active diving and snorkeling island.

Koh Chang — the large island near the Cambodian border, closest to Bangkok for overland travelers. White Sand Beach is the most developed strip; the interior is genuinely mountainous jungle with waterfalls. Less famous than the southern Gulf islands and often more affordable.

Koh Kood — the least-developed of the main Gulf islands, with no ATM at last check and a mostly boutique-resort crowd. Waterfalls into the sea. Worth it if you want real quiet.

Which Is Better for Diving — Andaman or Gulf?

Both coasts have world-class diving, but the character is different.

Andaman diving peaks from October through May. The Similan Islands (accessible from Khao Lak) offer exceptional visibility, diverse marine life including whale sharks and manta rays on the right seasonal conditions, and dramatic topography. Koh Phi Phi and Krabi have good local dive sites. The Mergui Archipelago across the border in Myanmar is accessible from some Andaman operators for liveaboard expeditions.

Gulf diving is most reliable from March through September on Koh Tao. The water is calmer during these months, visibility peaks, and the dive schools are at full capacity. Whale sharks appear at Koh Tao and Chumphon Pinnacle irregularly — there’s no guaranteed season, but March through May sees more sightings reported. The Gulf doesn’t have the dramatic wall diving of the Andaman’s deep-water sites, but the coral gardens around Koh Tao are healthy and the range of sites for all certification levels is excellent.

If you’re specifically planning around diving, the Best Time to Visit Thailand post has more granular month-by-month visibility and conditions data.

What If You Only Have Two Weeks and Want Both Coasts?

It’s doable but requires accepting that you’ll spend some of it on domestic flights rather than in the water.

A workable route during the Andaman dry season (November through April): Bangkok (2 nights) → Chiang Mai (3 nights) → fly to Phuket/Krabi (3 nights) → ferry to Koh Lanta or Koh Phi Phi (3 nights) → fly to Koh Samui via Bangkok (2 nights) → fly home from Samui or back through Bangkok. That gives you the Andaman at its peak plus Gulf islands before the northeast monsoon arrives.

During June through August, it inverts: Gulf islands first (Koh Tao for diving, Koh Phangan or Samui for beaches), then Bangkok, then north (Chiang Mai, Pai), and skip the Andaman entirely or plan only a day or two in Phuket accepting that beach days will be mixed.

The two-week itinerary covers three complete route options if you want the full breakdown with transport costs.

A Note on Accommodation Timing

For the Andaman coast, the best inventory on Koh Lanta and Krabi books quickly for December and January — book six to eight weeks out at minimum. Phuket has more capacity and stays available longer. Koh Phi Phi’s small supply of genuinely good places goes fast in high season; don’t expect to find acceptable beachside options if you search a week before arrival in January.

For the Gulf, Koh Tao’s dive-focused guesthouses are usually available closer to your dates because the diving clientele tends to book the dive package (which includes accommodation) through the dive school directly. Koh Samui’s resort inventory is large enough that finding rooms isn’t usually the challenge; finding value in peak season (January, Chinese New Year, European summer) is harder.

Search Andaman and Gulf island hotels on Agoda — they have comprehensive coverage of both coasts, including the smaller properties on Koh Kood and Koh Lanta that don’t always appear on global booking platforms.

For transport between islands, the 12Go ferry booking platform covers routes between Koh Samui, Koh Phangan, and Koh Tao (the Lomprayah and Seatran services), as well as Andaman ferry routes.

The Simple Cheat Sheet

There’s real overlap in February, March, and April — both coasts are dry and worth visiting, which is why this is the most popular and most expensive window for Thailand island travel.

For the full northern Thailand picture before you head south to the islands, see Chiang Mai & the North. Use the AI Trip Planner to map out the ferry connections, flight legs, and hotel timing across whichever coast fits your dates.

The islands are the reason most people come to Thailand, and they’re spectacular. The only planning mistake worth avoiding is picking the wrong coast for the wrong month. Everything else is navigable.