The best time to visit Thailand depends on which coast and which season, on the Thai Meteorological Department's own definitions: the rainy (southwest monsoon) season runs mid-May to mid-October, winter (northeast monsoon) mid-October to mid-February, and summer mid-February to mid-May, with April the hottest month. The cool, dry winter months bring the peak crowds, January 2025 saw 3.71 million international arrivals, the year's highest. The one fact that decides beach trips is regional: the Andaman coast (Phuket) is wettest in September at 361.2 mm, while the Gulf coast (Koh Samui) is wettest in November at 506.6 mm, so the two coasts take their turns. There is no official index that converts rainfall into savings, whatever older versions of this guide claimed.
The previous version of this article was titled "Best time to visit Thailand for 15% Lower Prices" and rested that headline on a "green-season value index" it attributed to Thai tourism boards, complete with a formula in which each 10 mm drop in monthly rain produced a 3 percent price reduction. No such index exists. The checks behind this rewrite found no price index, savings percentage or rainfall-price correlation published by the Tourism Authority of Thailand, the Ministry of Tourism and Sports or the Thai Meteorological Department, and the old article's own two worked examples did not follow its stated formula consistently. It also claimed PromptPay gives "up to 5% off restaurant bills", when the Bank of Thailand's own material describes PromptPay as payment infrastructure with no merchant discounts attached, and it priced night-market meals at an invented average of 40 baht. What follows keeps the useful question, when should you go, and answers it only from sources that exist: TMD for climate, the Bank of Thailand and the Ministry for arrivals and payments, UNESCO and the municipalities for the festival calendar.
Thailand's seasons, as TMD defines them
| TMD season | Approximate span | What it means |
|---|---|---|
| Rainy (southwest monsoon) | Mid-May to mid-October | Abundant rain countrywide; the wettest period is August to September |
| Winter (northeast monsoon) | Mid-October to mid-February | The mild period; quite cold in December and January in upper Thailand; heavy rain continues on the Gulf (east) coast, especially October to November |
| Summer (pre-monsoon) | Mid-February to mid-May | The transitional hot period; April is the hottest month |
These are the Thai Meteorological Department's own boundaries, from its climate monograph The Climate of Thailand: the southwest monsoon brings warm, moist air from the Indian Ocean from May, and the northeast monsoon brings colder, drier air from the China anticyclone from October, except on the southern east coast, where the northeast monsoon is precisely what delivers the rain. Everything else in this guide hangs off that frame: the cool-dry "high season" of travel writing is TMD's winter, the "green season" is TMD's rainy season wearing a marketing name, and the shoulder months are simply the transitions TMD describes. Use the seasons to plan, and the station figures below to plan by place, because Thailand's regions do not share one rainy season.
The numbers by place (TMD station normals, 1981-2010)
Bangkok, the baseline
TMD's Bangkok Metropolis normals (1981-2010, the series TMD publishes; it issues no 1991-2020 station series) put the capital's wettest month in September at 334.3 mm, with October second at 292.1 mm, and its driest in December at 6.3 mm, with January at 13.3 mm, against an annual total of about 1,648 mm. Mean daily maxima run from 31.7 degrees in December to 35.4 degrees in April. Read as a traveller: the capital is at its most visitable from November to February, hot and building from March to May, and properly wet from June to October, with September the month the city's drainage earns its keep. Bangkok tolerates rain better than the beach regions do, since its sights are urban, but the same September that is merely wet in the capital is the Andaman coast's peak, as the next section shows.
Phuket and the Andaman coast, wettest in September
Phuket's TMD normals peak in September at 361.2 mm inside a wet stretch that runs May to October under the southwest monsoon, and bottom out in February at 23.9 mm. That single peak drives the Andaman calendar: Phuket, Krabi and the islands off them are at their driest and calmest from November to April, and at their wettest, with the roughest seas, from June to October. Boat trips to the offshore islands are the first casualty of the southwest monsoon, and some national-park islands close for part of it in a typical year. If the Andaman is the point of the trip, winter is not a preference, it is the season the coast is built for; if you go in the rainy months, go for the mainland, the lower-key atmosphere the arrivals figures below reflect, and days planned around the rain rather than despite it.
Koh Samui and the Gulf coast, wettest in November
Two months after Phuket peaks, Koh Samui peaks harder: TMD's Samui normals put November at 506.6 mm, wetter than Phuket's wettest month, with October at 309.8 mm and December still at 210.3 mm. This is TMD's stated exception in numbers: the northeast monsoon that dries the rest of the country delivers the Gulf coast's rain, heaviest from October to December. The mirror fact matters as much: during June to September, while the Andaman sits at 213 to 361 mm a month, Samui runs a relative lull at 111 to 124 mm a month. So the rainy-season beach answer is not "skip the beaches", it is "choose the Gulf": Samui, Phangan and Tao are the rational beach base for a June-to-September trip, and the irrational one for late October to December, which is the exact reverse of the single "Thailand rainy season" advice this article used to give.
Chiang Mai and the north, cool in winter, wet in late summer
Chiang Mai's rainfall peaks in August at 216.9 mm and September at 211.4 mm, and its winter is genuinely cool by Thai standards, with mean minima of 14.9 degrees in January and 15.7 in December on TMD's normals, against a Bangkok that never needs a jacket. The north's best window is TMD's winter, mid-October to mid-February: dry, cool in the mornings, and timed for the lantern festival described below. The north also carries one honest caveat no climate normal can settle: the late-winter and early-spring burning season in the wider region can bring haze to the north in February to April in some years, which is a current-conditions check before travel, not a fixed calendar fact. Chiang Mai anchors the northern end of this site's Southeast Asia budget gems guide, where its temples, markets and costs are covered alongside seven other cities.
The calendar that actually moves prices: festivals and crowds
No authority publishes a seasonal price index, but the Bank of Thailand's tourism indicators, compiled from the Ministry of Tourism and Sports, publish the thing prices follow: arrivals. In 2025 Thailand received 32.97 million international visitors; the peak month was January 2025 at 3,709,102 arrivals, and the quietest was September 2025 at 2,235,850, with May the second quietest. That is the honest version of the old article's claim: the cool-dry peak around December and January is when demand, and therefore hotel pricing power, is at its highest, and the rainy-season trough months are when a traveller has the most negotiating room and the emptiest sights. September's trough comes with September's rain, 334 mm in Bangkok and 361 mm in Phuket on the normals above, so the trade is real in both directions and no index is needed to describe it. Against that baseline, two festivals move their own local markets: Songkran, the Thai New Year, fixed on 13 to 15 April in the official holiday calendar and inscribed by UNESCO in 2023 on the Representative List of the Intangible Cultural Heritage of Humanity as "Songkran in Thailand, traditional Thai New Year festival", which fills April's heat with the country's biggest domestic travel surge; and Loy Krathong with Chiang Mai's Yi Peng, whose main night in 2026 is Tuesday 24 November, the full moon of the twelfth lunar month (Chiang Mai Municipality's Yi Peng programme runs 23 to 25 November, with its grand krathong parade on the 25th, the date some listings mistake for Loy Krathong Day itself). No official 2027 Loy Krathong date was published at the time of writing, and secondary sources disagree, so this guide prints none rather than guess.
Money and entry, stated correctly
PromptPay first, because the old article got it wrong in a way that could strand a reader. PromptPay is Thailand's national instant-payment infrastructure, launched in 2016 under the Bank of Thailand: it lets people transfer money and pay using a mobile number or national ID linked to a bank account, instead of an account number. The Bank of Thailand publishes a transfer-fee table for it (free below 10 baht) and nothing else that touches tourists' wallets; no BOT or TAT source ties PromptPay to any merchant discount, and the "up to 5% off restaurant bills" claim was fabricated. A visitor without a Thai bank account cannot register for PromptPay at all, because registration means linking an existing account; the published tourist routes are different ones: cross-border QR payment linkages that let some visitors pay Thai QR codes from their home banking apps, and, on the TAT side, TAGTHAi Easy Pay, which pairs a passport with a Kasikornbank PAY&TOUR card. Carry cash and a card as your defaults. Entry itself: the visa exemption now gives 30 days, reduced from 60 by the Royal Gazette announcement of 31 August 2026, in force since 15 September 2026, for the 60 listed nationalities, tourism only, with land entries capped at two per calendar year; an extension costs THB 1,900 at Immigration. The Thailand Digital Arrival Card is free and mandatory since 1 May 2025, completed only through Immigration's official site. And the long-discussed tourist entry fee is still not collected: the current proposal, a flat THB 450 endorsed by the National Tourism Policy Committee in August 2026, still needs Cabinet approval and Gazette publication before it could take effect, with officials talking about early 2027 at the soonest; anyone charging you a Thai entry fee today is not the Thai state.
Sources
- Thai Meteorological Department (tmd.go.th): The Climate of Thailand (Climatological Group, Meteorological Development Bureau, 2015), the three seasons and their boundaries (rainy mid-May to mid-October, wettest August to September; winter mid-October to mid-February; summer mid-February to mid-May, April hottest), and the monsoon mechanisms with the southern east coast exception. Station normals 1981-2010 (TMD's published series; it issues no 1991-2020 series): Bangkok Metropolis, September 334.3 mm, October 292.1, December 6.3, January 13.3, annual about 1,648 mm, mean maxima 31.7 (December) to 35.4 degrees (April); Phuket, September peak 361.2 mm, February 23.9; Ko Samui, November peak 506.6 mm, October 309.8, December 210.3, June to September 111 to 124; Chiang Mai, August 216.9 and September 211.4 mm, mean minima January 14.9 and December 15.7 degrees.
- Bank of Thailand (bot.or.th): Tourism Indicators (compiled from the Ministry of Tourism and Sports), 2025 international arrivals 32.97 million, January 2025 peak 3,709,102, September 2025 trough 2,235,850, May second quietest. Bank of Thailand on PromptPay: the national instant-payment infrastructure launched 2016, transfers and payments by mobile number or national ID linked to a bank account, its published transfer-fee table (free under THB 10), and the account-linking requirement (no registration without a Thai bank account; no merchant discounts published by BOT or TAT).
- UNESCO Intangible Cultural Heritage (ich.unesco.org): "Songkran in Thailand, traditional Thai New Year festival", inscribed 2023; the official holiday dates 13 to 15 April (Bank of Thailand holiday calendar, 2026).
- Loy Krathong and Yi Peng 2026: the full-moon night of the twelfth lunar month, Tuesday 24 November 2026, consistent with the Fine Arts Department's Sukhothai festival (15 to 24 November 2026, thailand.go.th) and Chiang Mai Municipality's Yi Peng programme of 23 to 25 November 2026 (announced 23 September 2026), grand krathong parade on 25 November; no official 2027 date published at the time of writing.
- Entry: the visa-exemption reduction to 30 days, Royal Gazette announcement of 31 August 2026 effective 15 September 2026 (60 nationalities, tourism only, land entries capped at two per calendar year); Immigration extension fee THB 1,900; the Thailand Digital Arrival Card, free and mandatory since 1 May 2025 via tdac.immigration.go.th; the entry-fee position: a flat THB 450 proposal endorsed by the National Tourism Policy Committee on 14 August 2026, public hearing closed 28 September 2026, Cabinet approval and Gazette publication still outstanding, not collected.
