TL;DR: The best time to visit Khao Lak is November through April, when the Andaman Sea is calm, rainfall is at its lowest, and the Similan Islands National Park is open. The one date that should anchor your planning: the Similans close every year from May 15 to October 15 (confirmed by Thai National Parks), so any trip built around diving or an island day trip needs to fall inside that window. December and January are the driest, busiest, and most expensive months. May through October is the southwest-monsoon low season, rain, rough seas, the Similans shut, and a meaningful share of resorts and dive shops closed, but it’s also the cheapest and quietest Khao Lak ever gets.
Khao Lak’s calendar is simpler than most Thai beach destinations because one hard closure decides most of it. Unlike Phuket or Koh Samui, where “best time to visit” is mostly about comfort and crowd tolerance, Khao Lak’s trip-defining question is whether the Similan Islands are open. This guide walks through what each month actually looks like on the Andaman coast, why the Similans closure matters more than the weather forecast, and which type of traveller should pick which window.
Every temperature and rainfall figure below comes from long-term climate averages, and the Similan Islands dates come from Thailand’s National Parks authority and confirmed 2025-2026 season reporting, all listed in the Sources section. Temperatures are in °C. Prices, where mentioned, are in Thai baht (THB) with US dollars in parentheses, converted at ฿33 = US$1 (July 2026). Once you’ve picked your dates, pair this guide with our guide to things to do in Khao Lak.
Month-by-month: weather, Similans, and crowds
| Month | Weather | Similans / sea | Crowds & price |
|---|---|---|---|
| January | Dry, sunny, ~29°C highs, lowest humidity | Open; calmest seas of the year | Peak season, busiest and priciest |
| February | Dry, driest month (~47mm) | Open; calm seas, excellent diving | Peak season, still very busy |
| March | Dry, warming, ~31°C highs | Open; seas starting to build slightly | High season, easing a little |
| April | Hot, humidity rising, scattered showers begin | Open until May 15; book early-April trips before closure | Shoulder season, prices softening |
| May | Rain increasing, humid, seas roughening | Closes May 15; last chance for a Similans trip is early May | Low season begins, prices drop fast |
| June | Rainy season underway, frequent showers | Closed; rough seas on the open Andaman crossing | Low season, quiet, cheap |
| July | Heavy rain likely, humid | Closed | Low season, cheap, many dive shops shut |
| August | Rainy, some sunny breaks between storms | Closed | Low season, quiet, cheap |
| September | Very wet (~352mm+), frequent storms | Closed; rough seas | Low season, cheapest, quietest |
| October | Wettest month (~352-398mm), tapering by late month | Closed until October 15; reopens mid-month | Low season easing into shoulder |
| November | Rain easing, seas calming | Open from November; early dry season | Shoulder, rising demand |
| December | Dry, cool by local standards, ~29-30°C highs | Open; calm seas, excellent diving | Peak season, busiest of the year |
Temperature and rainfall figures are long-term monthly averages from Climates to Travel, Weather and Climate, and Climate-Data.org; see Sources. Similan Islands dates reflect the standard annual closure confirmed by Thai National Parks and 2025-2026 season reporting. Individual years can vary, the park has occasionally closed a little early in poor weather, so treat exact reopening/closing dates as approximate until confirmed closer to travel.
Why does the Similan Islands closure matter so much for Khao Lak?
Because for a large share of visitors, the Similan Islands are the reason to come to Khao Lak in the first place. The Similan Islands National Park closes every year from May 15 to October 15, per Thai National Parks, a roughly five-month shutdown driven by the southwest monsoon making the open-water crossing unsafe for standard tour and liveaboard boats, plus giving reefs a recovery window after the dive season. For the 2025-2026 season, the park reopened on October 15, 2025, in line with the standard annual pattern. This isn’t a soft guideline that a private boat can quietly ignore, it’s an enforced park closure, so no legitimate operator runs Similans day trips or liveaboards during those five months, whatever a stray listing online might suggest.
If diving, snorkelling, or a Similans day trip is the point of your Khao Lak trip, this closure should be the first thing you check, before flights, before a hotel, before anything else. Outside those dates, Khao Lak itself stays open, but the specific experience most people picture, boat out to turquoise water and granite boulder islands, simply isn’t available.
Why is December to February the best window?
This stretch combines the year’s lowest rainfall with the calmest seas on the Andaman coast. January and February are the driest months, averaging roughly 47-60mm of rain each, well below the wet-season months’ 300mm-plus, per long-term climate averages. Daytime highs sit in a narrow, manageable 29-31°C range all year, so heat isn’t really the variable, sea state and rain are, and both are at their best in this window. It’s also squarely inside the Similans’ open season, so diving, snorkelling, and day trips run on their full schedule with the best visibility of the year.
The trade-off is that everyone else has the same idea. December and January are Khao Lak’s peak season: the highest hotel rates, the busiest dive boats, and the most booked-out restaurants and beach resorts of the year. If you want dry-season conditions with a bit more breathing room, late November or February land inside the same weather window with somewhat less competition for rooms and boat spots than the December-January core.
What happens in Khao Lak during low season (May-October)?
This is the honest part most tour-operator sites gloss over. From roughly May through October, Khao Lak shifts into a genuinely different mode:
- The Similan Islands are closed, full stop, for the entire window (May 15-October 15).
- Seas roughen with the southwest monsoon, and some Khao Lak beaches post red-flag or rough-water warnings during storms.
- Rain is frequent and sometimes heavy, peaking in September and October, both averaging 350mm-plus of rainfall over 20-plus rainy days.
- A meaningful share of smaller resorts, restaurants, and dive shops close for some or all of the low season, since their business depends on Similans traffic and dry-season crowds. Larger hotels generally stay open, but the range of what’s running narrows.
- Prices drop substantially, and the beaches, roads, and restaurants that do stay open are noticeably quieter.
None of this makes Khao Lak a bad destination in low season, it just makes it a different one. It suits travellers on a tight budget, those who want a quiet beach break with no diving agenda, and people genuinely comfortable with tropical rain. It does not suit anyone whose trip depends on the Similans, and if that’s you, no low-season date works, regardless of specific weather that week.
Which window should you pick?
- Divers and Similans day-trippers: book strictly inside October 15-May 15, and lean toward December-February for the calmest seas and best visibility.
- First-time visitors wanting the full experience: December to February. Driest, calmest, everything open, at the cost of peak prices and crowds.
- Budget travellers not chasing diving: the low season, June-August, before the September-October rain peak, gives real savings and quiet beaches, with the understanding that the Similans and some seasonal businesses are off the table.
- Shoulder-season value seekers: November or late April/early May sit right at the edges of the open season, often with softer prices than the December-January peak and still-workable weather.
- Anyone who wants to avoid the wettest weeks outright: skip September and October, the two wettest months of the year on the Andaman coast.
Our recommended window
If your schedule is flexible, book Khao Lak for December through February. It’s the reliable core of the dry season: lowest rainfall, calmest seas, and the Similan Islands running their full season, which is what most people are actually building a Khao Lak trip around. If you want similar conditions with a little less competition for rooms and boats, late November or February are strong alternatives just outside the absolute peak.
If your dates are fixed and fall between May and October, go in with realistic expectations rather than fighting them: the Similans are not an option in that window no matter how the trip is planned, seas will be rougher, rain is likely on most days, and it’s worth confirming ahead of time which specific resorts, restaurants, and dive shops you’re counting on will actually be open. Treat it as a quieter, cheaper, land-and-beach version of Khao Lak rather than a discounted version of the December trip.
Once you’ve locked your dates, pair this guide with things to do in Khao Lak for the pillar overview, our breakdown of Khao Lak’s beaches for where to base yourself, and the best time to visit Thailand for how the Andaman coast’s calendar compares to the Gulf coast and the north. If island-hopping beyond the Similans is on your list, our guide to Thailand’s best islands covers the wider options. And once your dates are set, check what’s on around Khao Lak and the Andaman coast for the week you’re actually there.
Sources
- Thai National Parks: Mu Ko Similan National Park: official annual open/closed dates (October 15-May 15), daily park hours
- 5 Star Marine Phuket: Similan Islands Are Closed May 15th-October 15th, 2025: confirmed 2025 closure date and season pattern
- 5 Star Marine Phuket: Similan Islands Will Close May 15th 2026: confirmed 2026 closure date and season overview
- The Phuket Express: Similan Islands in Phang Nga Reopens: confirmed October 15, 2025 reopening for the 2025-2026 season
- Weather2Travel: Khao Lak Climate: month-by-month temperature and rainfall guidance, dry/rainy season pattern
- Weather and Climate: Khao Lak Monthly Averages: monthly high/low temperatures, rainfall totals, rainy-day counts
- Climate-Data.org: Khao Lak Climate: annual and monthly temperature and rainfall averages
- Holiday Weather: Khao Lak Averages: monthly temperature and rainfall reference
- Xe.com: USD/THB Currency Converter: exchange rate reference, July 2026