The Andaman islands (Phuket, Phi Phi, Lanta, Lipe) are at their best from about December to March and wettest in September and October. The Gulf islands (Samui, Phangan, Tao) have the opposite problem: they are fairly dry from February to August but wettest from October to December, with November the peak. The eastern islands near Bangkok (Samet, Chang, Kood) are dry-season islands, best from November to April.
The reason is the two monsoons. The southwest monsoon (about mid-May to October) blows rain and swell onto the west-facing Andaman coast. The northeast monsoon (from late October) dries out most of Thailand but pushes rain onto the east side of the southern peninsula, where the Gulf islands sit. If you know which sea an island is in, you already know most of what you need. Our overview of the Thai islands introduces each group; this page is only about timing.
Island seasons at a glance
- Andaman Sea, best
- December to March
- Andaman Sea, wettest
- September and October (Phuket town 350 and 336 mm)
- Gulf of Thailand, best
- February to August, with February and March often the driest
- Gulf of Thailand, wettest
- October to December (Koh Samui 445 mm in November)
- Eastern islands, best
- November to April
- Seasonal closures
- Similan and Surin closed 15 May–15 October; Maya Bay closed 1 August–30 September
Average rainfall at the island stations
These are the Thai Meteorological Department's 30-year averages (1991–2020) in millimeters per month. Most islands have no weather station of their own, so for some we use the nearest mainland station; it gives the pattern, not the exact figure.
| Month | Phuket town | Ko Lanta | Satun (for Koh Lipe) | Ko Samui | Chumphon (for Koh Tao) | Rayong (for Koh Samet) | Trat (for Koh Chang) |
|---|---|---|---|---|---|---|---|
| Jan | 50 | 31 | 44 | 127 | 97 | 29 | 51 |
| Feb | 24 | 24 | 41 | 64 | 52 | 32 | 82 |
| Mar | 81 | 69 | 134 | 116 | 110 | 71 | 140 |
| Apr | 135 | 122 | 210 | 83 | 93 | 85 | 196 |
| May | 237 | 228 | 229 | 132 | 171 | 191 | 420 |
| Jun | 249 | 286 | 190 | 134 | 163 | 178 | 777 |
| Jul | 240 | 289 | 230 | 117 | 195 | 177 | 1,024 |
| Aug | 309 | 342 | 278 | 102 | 199 | 128 | 970 |
| Sep | 350 | 319 | 334 | 118 | 179 | 269 | 816 |
| Oct | 336 | 298 | 344 | 295 | 247 | 196 | 338 |
| Nov | 178 | 148 | 218 | 445 | 259 | 44 | 85 |
| Dec | 82 | 65 | 118 | 263 | 173 | 8 | 28 |
Rain is only half the story on an island. On the Andaman side the southwest monsoon also brings swell, strong currents and rough crossings, which matter more for boat trips and swimming than the rain itself. For full-country monthly figures, including Bangkok and the north, see Thailand's weather month by month.
The Andaman islands: Phuket, Phi Phi, Krabi, Lanta and Lipe
From December to March the Andaman Sea is usually calm and clear, which is why this is the main season for Koh Phi Phi, Koh Lanta, the Krabi islands and the Trang islands. Phuket town averages just 24 mm in February. It is also the busiest and most expensive time.
April is hot and still fine. From May the southwest monsoon arrives: Phuket town jumps from 135 mm in April to 237 mm in May, and the sea gets rough on west-facing beaches. Lifeguards on Phuket's west coast raise red flags when swimming is unsafe, and the UK Foreign Office warns that people have drowned in strong riptides, especially in monsoon season.
Two marine areas close completely. The Similan and Surin Islands are open only from 15 October to 15 May, and Maya Bay on Koh Phi Phi Le closes every year from 1 August to 30 September. Further south, Koh Lipe has fewer boat connections in the low season, so check the current timetable before you plan a trip there between June and mid-October.
The Gulf islands: Koh Samui, Koh Phangan and Koh Tao
The Gulf islands work almost in reverse. Koh Samui averages only 102–134 mm a month from June to August, while Phuket gets 240–309 mm, which makes the Gulf the natural choice for a beach holiday in the European and North American summer. February and March are also good, and February (64 mm) is Samui's driest month.
The wet season starts in October and peaks in November, when Samui averages 444.6 mm, the highest monthly figure at any of the island stations above except Trat. December is still wet (263 mm), and January has showers (127 mm on about 11 days). Koh Phangan and Koh Tao follow the same pattern; the nearest mainland station, Chumphon, peaks in October and November too.
Official sources describe the Gulf season slightly differently. The UK Foreign Office says the rainy season "in Koh Samui and south-east Thailand" is "usually from November to March". The TMD averages show that October is already wet and that February and March are among the driest months, so we treat October to December as the wet season and January as a transition.
From July to October, one more thing to know on the Gulf side: Thai marine researchers reported in 2019 that they had frequently observed box jellyfish off Koh Samui, Koh Phangan and Koh Tao in those months. See our guide to box jellyfish in Thailand before you swim.
The eastern islands: Koh Samet, Koh Chang and Koh Kood
The islands east of Bangkok follow the general Thai pattern of a dry season from November to April and rain from May to October, but in very different amounts. Koh Samet, off Rayong, is one of the drier islands: the Rayong station averages 1,407 mm a year, with December at just 7.5 mm. That, plus its short distance from Bangkok, makes it a reasonable weekend option even in the rainy months.
Koh Chang and Koh Kood are a different case. The Trat coastal station averages 4,927 mm a year, with more than a meter of rain in July and close to a meter in August. Visit them between November and April.
Where to go in October, November and December
These three months cause most of the confusion, because they are "high season" for most of Thailand but not for every island.
- October: the hardest month for islands. Both seas are wet: Phuket town averages 336 mm, Samui 295 mm and Satun 344 mm. The Similan Islands reopen on 15 October. No island group is reliably dry; Koh Samet (196 mm) is the least wet of the stations above. On the Andaman side the southwest monsoon usually gives way in the second half of the month, so late October tends to be the better half.
- November: go Andaman. Phuket drops to 178 mm and keeps improving through the month, and Koh Lanta averages 148 mm. Avoid the Gulf: it is Samui's wettest month. The eastern islands are good again (Rayong 44 mm, Trat 85 mm).
- December: the Andaman islands (Phuket 82 mm, Lanta 65 mm) and the eastern islands (Rayong 7.5 mm, Trat 28 mm) are in season. Samui is still wet at 263 mm; if you want the Gulf, wait for February.
A November trip to Koh Samui is the most common timing mistake. "November to April is the dry season" is true for Bangkok and the Andaman coast, but November is the single wettest month on Samui. If your flights are fixed, swap Samui for Koh Lanta, Krabi or Phuket.
Not sure which island suits you beyond the season? Our guide to choosing the right Thai island compares them by budget and style. For the low-season practicalities, such as boat cancellations and buffer days, read what the rainy season is really like.
Frequently asked questions
Which Thai islands are best in July and August?
The Gulf islands: Koh Samui, Koh Phangan and Koh Tao. Samui averages about 100–120 mm of rain a month in July and August, less than half of Phuket's total. The Andaman side is in its southwest monsoon, with rough seas and red-flag days.
Is November a good month for Koh Samui?
No, it is the wettest month of the year there, with an average of 444.6 mm according to the Thai Meteorological Department. November is a good month for the Andaman islands such as Koh Lanta and Phuket, and for Koh Samet and Koh Chang.
When are the Similan Islands open?
From 15 October to 15 May. The Department of National Parks closes the Similan and Surin marine parks to visitors and overnight stays every year from 15 May to 15 October during the southwest monsoon. The 2026–27 season is due to open on Thursday 15 October 2026; confirm the first operating day with your boat operator.
Can I visit Maya Bay in September?
No. Maya Bay on Koh Phi Phi Le closes every year from 1 August to 30 September to let the reef and beach recover and for visitor safety in the monsoon. It reopened on 1 October 2026. Swimming in the bay is still not allowed.
Sources
- Thai Meteorological Department – Climatological Data of Thailand 1991–2020 (checked October 2026)
- Tourism Authority of Thailand – The best time to visit Thailand's beaches and islands (checked October 2026)
- UK Foreign, Commonwealth and Development Office – Thailand: safety and security (checked October 2026)
- Department of National Parks – Maya Bay seasonal closure 2026 (checked October 2026)
- Nation Thailand – Similan and Surin islands close for the monsoon season (checked October 2026)
- Nation Thailand – Tourists warned of box jellyfish at Surat Thani's popular islands (2019) (checked October 2026)