Best Thai Islands by Month: Andaman vs Gulf Seasons
The southwest monsoon runs from mid-May to mid-October. The northeast monsoon then runs to mid-February, and the south keeps heavy rain until December, with the Gulf side wetter than the Andaman side. Similan and Surin close from 16 May to 14 October.
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Gulf coast and Andaman coast by month
Thailand has two island coasts with opposite rainy periods. The Andaman Sea is on the west side of the south and the Gulf of Thailand on the east side. The west coast covers Ranong, Phang Nga, Phuket, Krabi, Trang and Satun. The east coast includes Chumphon and Surat Thani.
The Thai Meteorological Department splits the year into three seasons. The hot season runs from mid-February to mid-May. The rainy season runs from mid-May to mid-October. The northeast monsoon blows from about mid-October to mid-February.
What each monsoon does
The southwest monsoon covers Thailand from mid-May to mid-October. It brings heavy cloud and rain, most of all along the coasts and on mountains facing the wind.
The northeast monsoon takes over from about mid-October. The south then stays rainy, especially the east side, because this monsoon carries moisture from the Gulf of Thailand.
Heavy rain in the south continues until December and often turns very heavy and floods. The east side of the south gets more rain than the west side.
Which coast suits which period
| Period | Season | Rain pattern |
|---|---|---|
| Mid-October to December | Northeast monsoon | South rainy to December, east side wetter |
| January to mid-February | Northeast monsoon | Monsoon runs to mid-February |
| Mid-February to mid-May | Hot season | Season runs to mid-May |
| Mid-May to mid-October | Southwest monsoon | Rain on coasts and windward hills |
From mid-October to December the Andaman side gets less rain than the Gulf side. That is the window when the Andaman marine parks reopen.
From mid-May to mid-October the southwest monsoon rains on the coasts. The Andaman marine parks close for much of that window, as the table below shows.
Month by month: Gulf or Andaman
The Thai Meteorological Department’s averages for 1991 to 2020 show the two coasts swap places. The Koh Samui station stands for the Gulf and the Phuket station for the Andaman Sea. The drier coast of the two is the pick for that month.
| Month | Gulf side: Koh Samui station | Andaman side: Phuket station | Go here |
|---|---|---|---|
| January | 126.8 mm, 11.1 rainy days | 49.9 mm, 5.6 rainy days | Andaman. Nothing on the closure list is shut. |
| February | 64.3 mm, 5.6 rainy days | 24.0 mm, 3.4 rainy days | Andaman. The driest month on either coast. |
| March | 115.8 mm, 5.9 rainy days | 80.7 mm, 7.5 rainy days | Andaman, though Gulf rain is also moderate. |
| April | 82.9 mm, 8.1 rainy days | 135.1 mm, 11.7 rainy days | Gulf. Andaman rain climbs. |
| May | 131.6 mm, 14.0 rainy days | 236.9 mm, 18.9 rainy days | Gulf. Similan and Surin close on 16 May. |
| June | 133.6 mm, 13.1 rainy days | 249.4 mm, 18.6 rainy days | Gulf. Koh Rok and Tarutao close on 1 June. |
| July | 117.0 mm, 14.3 rainy days | 240.2 mm, 19.1 rainy days | Gulf. Andaman has about twice the rain. |
| August | 102.2 mm, 14.5 rainy days | 308.7 mm, 19.5 rainy days | Gulf. Maya Bay and Phang Nga Bay sites close on 1 August. |
| September | 117.5 mm, 15.0 rainy days | 350.4 mm, 21.7 rainy days | Gulf. Andaman is at its wettest. |
| October | 294.9 mm, 19.3 rainy days | 336.0 mm, 22.7 rainy days | Neither: both coasts are at their wettest. Andaman parks reopen on 15 October. |
| November | 444.6 mm, 18.7 rainy days | 178.2 mm, 15.9 rainy days | Andaman. Gulf rain peaks. |
| December | 263.1 mm, 14.2 rainy days | 82.0 mm, 10.3 rainy days | Andaman. Ang Thong sites on the Gulf close until 15 December. |
A rainy day is a day with 0.1 mm or more of rain. Each station is one point on one island, and nearby beaches differ.
The Andaman coast is the better pick from November to March. The Gulf coast is the better pick from April to September. October is wet on both coasts.
Marine park closure dates
Every marine park publishes its closed periods in a yearly Gazette announcement. The 2026 annex lists these island parks.
| Park or site | Closed |
|---|---|
| Similan Islands | 16 May to 14 October |
| Surin Islands | 16 May to 14 October |
| Maya Bay, Loh Samah, Lo Ko bay (Phi Phi) | 1 August to 30 September |
| Phang Nga Bay: Koh Lawa Yai, Ao Khian, Koh Boi Yai | 1 August to 30 September |
| Koh Rok (Lanta park) | 1 June to 30 September |
| Koh Ha, Hin Muang, Hin Daeng (Lanta park) | 1 August to 30 September |
| Tarutao: Adang, Rawi and listed islands | 1 June to 30 September |
| Hat Chao Mai: Koh Cheuk, Koh Waen | 1 June to 30 September |
| Ang Thong Islands (Gulf) | 1 November to 15 December |
| Koh Samet: Koh Kudi | 1 July to 15 August |
| Koh Samet: Koh Talu | 16 August to 30 September |
Similan and Surin
The Similan Islands close from 16 May to 14 October. The Surin Islands close on the same dates. The Similan park welcomed visitors again on 15 October 2026.
Phi Phi, Phang Nga Bay and Lanta
Maya Bay and its neighbouring bays close from 1 August to 30 September. Phang Nga Bay’s closed sites follow the same dates. Lanta’s Koh Rok closes from 1 June to 30 September.
Gulf parks
The Gulf side has fewer closures. Ang Thong closes some sites from 1 November to 15 December. Koh Samet closes Koh Kudi and Koh Talu one after the other between 1 July and 30 September.
Choosing by month
Mid-October to December
The Andaman marine parks have opened for the season, including Similan, Surin, Phi Phi, Lanta and Phang Nga Bay. Rain is lighter on the Andaman side than on the Gulf side in this window. Ang Thong closes some sites from 1 November to 15 December.
January to mid-February
The northeast monsoon still blows until mid-February. No Andaman park on the list above is closed in these months. Heavy rain in the south runs to December.
Mid-February to mid-May
This is the hot season. Similan and Surin close on 16 May.
Mid-May to mid-October
The southwest monsoon rains on the coasts. Similan and Surin are closed. Koh Rok, Tarutao and Hat Chao Mai are closed from 1 June. Phi Phi’s Maya Bay, Phang Nga Bay’s listed sites and Koh Ha close from 1 August.
Three sample trips
Late October
Similan opened on 15 October 2026. Surin’s closed period ended on 14 October. The northeast monsoon has begun, and the Gulf side gets more rain than the Andaman side.
Late July
Similan and Surin are closed. Koh Rok has been closed since 1 June. Maya Bay is still open, because its closure starts on 1 August.
Early January
The northeast monsoon is still blowing. None of the closures in the table covers early January. The south’s heavy rain period ends in December.
Dates that move
The start and end of both monsoons can vary from the normal dates in any year. Opening of some sites and accommodation can change with the weather. The 2026 Maya Bay closure ran from 1 August to 30 September 2026.
Before you book
- Check the park’s closed period for your sailing date, as listed above.
- For a Similan or Surin trip, plan between 15 October and 15 May.
- For Phi Phi, avoid August and September if Maya Bay is the goal.
- Allow slack at the edges of a monsoon window, since the rainy season can start 1 to 2 weeks early or late.
Not confirmed: The month table uses one weather station per coast. The rainy season can start 1 to 2 weeks early or late; check the park announcement for your sailing date.
Common questions
Which Thai island coast is best in December?
The Andaman side. The south keeps heavy rain until December, and the Gulf side gets more of it than the Andaman side.
When is the rainy season in Thailand?
The rainy season runs from mid-May to mid-October. The south stays rainy until December.
When do the Similan Islands close?
They close from 16 May to 14 October. The park opened for the 2026 season on 15 October.
When is Maya Bay closed?
Maya Bay, Loh Samah and Lo Ko bay close from 1 August to 30 September.
Are the Surin Islands open in July?
No. The Surin Islands are closed from 16 May to 14 October.
Which provinces are on the Andaman coast?
Ranong, Phang Nga, Phuket, Krabi, Trang and Satun.
Which coast is drier in August, Gulf or Andaman?
The Gulf. Koh Samui averages 102.2 mm in August against 308.7 mm at Phuket.
Is the Gulf of Thailand dry when the Andaman is wet?
The southwest monsoon rains hardest along coasts and windward mountains. Heavy rain reaches the Gulf side from mid-October, when the northeast monsoon arrives.
Do monsoon dates change each year?
Yes. The start and end of both monsoons can vary from the normal dates.
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Comments (5)
AstroTommy
I recommend Koh Samet, it's a popular islands weekend holiday spot for locals from Bangkok. The island has a national park on it, it's a few hours by bus from Bangkok Ekamai station
peteyy_cz
just don’t go to Phuket, fly to Krabi that’s like a 2 hour flight from over there. I’d recommend Ao Nang beach find a cheap hotel and hang around the beach for a few days. You can go to Phi Phi by boat from there for a day trip.
tallwhiteguycebu
I second this, just fly to Krabi and stay in Ao Nang. They have these long tail boats that will take you to different beaches for 200 baht round trip
thehotflashpacker
I just spent 16 days at Koh Lanta. (Khong Dao beach). Had a nice pool, super easy swimming beach. Not so expensive excursions by motor boat to good snorkel spot, phi phi, and emerald cave. Small night market, lots of chill bars and restaurants to chose from. Easy ride from Krabi airports (2 hrs).
cliff0217
Less about the type of beach, more about weather. Also, you didn't state the length of time. You need to go in with expectation that it will rain, and to be flexible when it does. I think Aug/Sept, Samui has least amount of rain. I suggest renting a car. To me, Samui is compact, has good hiking spots, easy to find food, and more options for mid-level/high-end hotels - ultimately it attracts a more mature traveler. You'll run into partygoers because it's a docking point for Phangan, but for the most part, prices out many tourists. Some would say Hua Hin, but to me, it's a weekend trip (if that's what you want, then great). Hope this helps!