Short answer
Choose the Andaman coast (Phuket, Krabi) from November to February and the Gulf coast (Koh Samui, Koh Tao) from April to September; March is a tie and October is wet on both. The 1991 to 2020 normals show why: in December Phuket Airport gets 86 mm of rain and Koh Samui 263 mm, while in August Phuket gets 378 mm and Samui only 102 mm. The calendars also set the parks: the Similan Islands are open from 15 October to 14 May, and Ang Thong is closed from 1 November to 15 December 2026.
Why the two coasts have opposite rainy seasons
Southern Thailand is a peninsula with a mountain spine, and the Thai Meteorological Department (TMD) describes the two sides as separate regions: the west coast, facing the Andaman Sea, and the east coast, facing the Gulf. The south-west monsoon, from about mid-May to mid-October, brings heavy rain to the west coast, which peaks in September. The north-east monsoon then brings mild weather and abundant rain to the east coast, with a peak in November.
TMD's 1981 to 2010 averages show the size of the difference. The west coast receives about 1,841 mm in the rainy season and 465 mm in winter, while the east coast receives 680 mm in the rainy season and 828 mm in winter. In other words, the Gulf side of the south gets more rain in the cool months than in the monsoon months, which is the opposite of what most visitors expect.
Phuket and Koh Samui, month by month
The 1991 to 2020 normals put numbers on it. Phuket Airport records 32 mm in February and 86 mm in December, rising to 278 mm in May, 378 mm in August and 399 mm in September. Koh Samui records 64 mm in February, 83 mm in April and 102 to 134 mm a month from May to September, then 295 mm in October, 445 mm in November and 263 mm in December. Phuket's yearly total is about 2,664 mm, Samui's about 1,994 mm.
Set side by side, the picture is simple. From November to February Phuket is the drier station: 219 against 445 mm in November, 86 against 263 in December, 66 against 127 in January. March is level at about 117 mm each. From April to September Samui is clearly drier: 144 mm against 83 in April, and 260 to 399 mm against 102 to 118 in July, August and September. October is wet on both sides, at 371 mm at Phuket Airport and 295 mm on Samui. Rain days follow the same pattern.
The sea and the marine parks
The parks follow the weather. On the Andaman side the Department of National Parks declared the marine park season open on 15 October 2026, naming the Similan Islands, Surin Islands, Phi Phi, Lanta and Ao Phang Nga. Our Similan day runs from 15 October to 14 May, and Maya Bay is closed every year from 1 August to 30 September. The UK Foreign Office warns of strong riptides along the coast during the monsoon, so the rainy months need extra care.
On the Gulf side, Ang Thong National Marine Park is closed to visitors from 1 November to 15 December 2026, and our Ang Thong tour runs before and after those dates. TMD's seasonal outlook of 15 October 2026 warned of heavy rain along the Gulf coast from Chumphon southwards in November and December, with waves of two to four metres possible, according to The Thaiger. The Koh Tao crossing, our operator says, is roughest in the wetter months, mostly late October to December.
Island day trips compared: what each coast offers
On the Andaman side our days are the Phi Phi and Maya Bay speedboat tour from $75 per person (about 8.5 hours), the James Bond Island and Phang Nga Bay canoe tour from $85 (about 8.5 hours), and the Similan Islands day from $105 (12 to 13 hours door to door, passport copies needed). Phuket's tours are long days on shared speedboats of up to 35 guests.
On the Gulf side the Ang Thong Marine Park tour is from $50 per person (about 8 to 9 hours), the Pig Island day from $50 (5 to 6 hours) and the Koh Tao and Nang Yuan day from $55 (about 8.5 hours). Prices are a guide to the day's length and cost, not to quality: choose by what you want to see, using our Phuket or Koh Samui comparison.
If your dates are fixed: four common cases
Christmas and New Year: choose the Andaman coast. December gives 86 mm at Phuket Airport against 263 mm on Samui, the Similans are open, and Ang Thong is closed until 15 December. July and August: choose the Gulf, where Samui has about 117 and 102 mm against 260 and 378 mm at Phuket, although Maya Bay is closed in August. April: Samui has 83 mm against 144 mm, but both are hot, as TMD calls April the hottest month.
November is the hard case for the Gulf. Samui's normal is 445 mm on 16.5 days, TMD's 2026 outlook expects heavy rain there, and Ang Thong is closed, so the Andaman coast (219 mm at Phuket Airport) is the better pick. If you want both coasts in one trip, the Gulf suits January to April before the rains, then the Andaman coast from late October or November. Check each park's dates before you fix a boat day.
Our Ang Thong day, and when it runs
The Ang Thong speedboat day is from $50 per person and takes about 8 to 9 hours, with hotel pickup on Koh Samui between about 7:30 and 8:30 am and the boat leaving the pier in the north of the island at about 9:00. You see the Emerald Lake, climb to the viewpoint over 42 islands, and have a Thai buffet lunch on the beach. The national park fee is paid on the day and is not in the price, so bring cash.
The viewpoint climb is steep, with ropes, and takes about 15 minutes; children and older guests can stay on the beach. Swimming in the Emerald Lake is not allowed. The tour runs before 1 November and again from 16 December 2026. If your dates fall inside the closure, send them to us and we tell you which other Samui days are running, and if the sea is unsafe the operator moves a trip to another date or refunds it.
Practical checklist
- Match the month to the coast first: Andaman from November to February, Gulf from April to September.
- Check each park's dates: Similan 15 October to 14 May, Ang Thong closed 1 November to 15 December 2026.
- Avoid the Gulf islands in November, when Koh Samui's normal is about 445 mm.
- Bring cash for the Ang Thong park fee and shoes with grip for the viewpoint.
- Keep a spare day for any sea itinerary, because boats move or cancel when the sea is unsafe.
Questions and answers
Which is better, the Andaman coast or the Gulf of Thailand?
Neither is better in every month. The Andaman side is drier from November to February and the Gulf side from April to September. March is level, so choose by your dates and the park calendar.
Is Koh Samui or Phuket better in December?
Phuket. The 1991 to 2020 normals give 86 mm at Phuket Airport in December against 263 mm on Koh Samui, and Ang Thong is closed until 15 December 2026.
Is Koh Samui or Phuket better in July and August?
Koh Samui is drier, with about 117 mm in July and 102 mm in August, against 260 and 378 mm at Phuket Airport. Maya Bay is closed in August and September.
When is Ang Thong Marine Park open?
Our tour runs before 1 November and again from 16 December 2026. The park is closed to visitors from 1 November to 15 December 2026. Check the park's own notices before you book near those dates.
Can I visit both coasts in one trip?
Yes. The Gulf side suits January to April and the Andaman side from November to April, so a trip in late January to April can cover both. Allow time for the transfer and a spare day for the sea.