May is a transition month. TMD describes the first half as usually sweltering, with thunderstorms and sometimes hail, and the start of the rainy season in the second half. Below are Thai Meteorological Department (TMD) normals for nine stations, what May 2026 looked like, sea safety, national park closures and the holidays that fall in May 2027. Checked on 25 October 2026.
May weather by region: TMD normals
These are 1991-2020 normals from the TMD report; the Krabi station (48 563) has a shorter 1994-2020 record. A rainy day means 0.1 mm or more. Khao Lak uses the nearest TMD station, Takua Pa (individual beaches may differ), Pattaya the Phatthaya station, Koh Chang the Trad station on the mainland. TMD has no stations on Koh Phangan or Koh Tao, so the Ko Samui station is the nearest reference for them.
| Region, TMD station | High, °C | Low, °C | Rain, mm | Rainy days | Hotel occupancy May 2025, % |
|---|---|---|---|---|---|
| Phuket (Phuket) | 33.2 | 26.1 | 236.9 | 18.9 | 73.95 |
| Khao Lak (nearest station Takua Pa) | 32.6 | 24.7 | 430.9 | 22.4 | 72.44 (Phang Nga) |
| Krabi (Krabi, 1994-2020) | 33.0 | 23.5 | 210.1 | 17.5 | 73.36 |
| Koh Samui (Ko Samui) | 32.9 | 25.7 | 131.6 | 14.0 | 77.88 (Surat Thani) |
| Pattaya (Phatthaya) | 32.7 | 26.6 | 121.1 | 11.0 | 79.44 (Chon Buri) |
| Hua Hin (Hua Hin) | 33.7 | 26.3 | 95.5 | 12.3 | 77.01 (Prachuap Khiri Khan) |
| Bangkok (Bangkok Metropolis) | 35.1 | 26.9 | 216.8 | 15.0 | 70.29 |
| Chiang Mai (Chiang Mai) | 34.7 | 24.2 | 167.5 | 14.8 | 67.94 |
| Koh Chang (nearest station Trad, mainland) | 32.8 | 24.8 | 419.9 | 21.9 | 62.87 (Trat) |
Hotel occupancy from the Ministry of Tourism and Sports (MOTS) is by province, not by resort. A normal is a 30-year average, and any single year can be much drier or wetter. Hotel occupancy in Phuket was 74.0% in May 2025 against 86.4% in January.
How May behaves: TMD’s description and May 2026
TMD’s 3-month outlook issued on 29 April 2026 describes May from the 30-year pattern. The first half is usually sweltering, with thunderstorms and sometimes hail. In the second half, as the rainy season starts, temperatures fall and rain increases while the wind turns to the southwest monsoon. TMD adds that tropical cyclones forming in the Andaman Sea and the Bay of Bengal may move toward the western side of Thailand.
May 2026 shows how far a single year can stray from the normal:
- a high of 42.1 °C in Mueang district, Lampang, on 4 May
- the start of the 2026 rainy season recorded by TMD on 15 May
- rain across Thailand 21% above normal, and 90% above normal (271.9 mm more) on the western coast of the south
- 138.0 mm in one day at Takua Pa on 20 May, and flooding in Phuket during 18-20 May
Where to go in May
Gulf islands: Koh Samui, Koh Phangan, Koh Tao. The Samui station records 131.6 mm on 14.0 days, under a third of Takua Pa’s 431 mm and just over half of Phuket’s. TMD explains that the western coast of the south gets its rain in the southwest monsoon and the eastern (Gulf) coast in the northeast monsoon, which is why the Gulf is the wetter side only later in the year. More in the Koh Samui holiday guide.
Hua Hin and Pattaya. The driest of the nine stations: Hua Hin 95.5 mm on 12.3 days and Pattaya 121.1 mm on 11.0 days, with highs of 32.7-33.7 °C. Chon Buri province, where Pattaya sits, was 79.4% full in May 2025, close to its 82.1% in January.
Phuket, Krabi, Khao Lak. Rain climbs from April’s 135 mm to 237 mm at Phuket (18.9 rainy days against 11.7), and from 222 mm to 431 mm at Takua Pa. West-facing beaches begin to get waves and the Similan Islands close on 16 May. Details in Phuket in the rainy season.
Bangkok and Chiang Mai. The hottest places in the table: average highs of 35.1 °C in Bangkok and 34.7 °C in Chiang Mai, with 217 mm and 168 mm of rain. The Pollution Control Department (PCD) lists January-May as a PM2.5 watch period for the north, so check the air before outdoor plans: Chiang Mai smoke season.
Koh Chang. The Trad station on the mainland records 420 mm on 21.9 days in May and 777 mm in June. It is a winter beach, see the Koh Chang guide.
Sea and swimming in May
The sea changes before the calendar does. TMD’s May summary says the southwest monsoon strengthened to rather strong at the end of May 2026. Rawai municipality’s page on Nai Harn, which also mentions the neighbouring Ao Sane bay, says that in the monsoon season from May to October the waves are very strong and not suitable for swimming, and Karon municipality describes the waves at Karon Beach as fairly strong. TAT explains the beach flags: red means do not swim, yellow means swimming is allowed but the risk is medium, green means calm conditions, and a red-yellow flag marks the area where lifeguards are on duty. TAT also says island-hopping and boat trips still run in the monsoon season but can be delayed or cancelled at the last minute, which is the captain’s decision.
We have no official wave heights for May, so go by the flag on the day. What to do if a current pulls you out is in red flags and rip currents, and calmer spots on Phuket are in Phuket beaches without waves.
National parks and islands in May
The Department of National Parks (DNP) publishes a closure table for national parks every year. The 2026 table is dated 7 January 2026 and the 2027 table is not out yet, so these are 2026 dates; confirm with your operator before booking.
| Park and sites | Closed | Source |
|---|---|---|
| Mu Ko Similan National Park, all tourist sites | 16 May-14 October | DNP park page, 2026 table |
| Mu Ko Surin National Park, all tourist sites | 16 May-14 October | DNP 2026 table |
| Ko Rok, Hin Daeng and Hin Muang (Mu Ko Lanta National Park) | 16 May-15 November | DNP park page |
| Ao Khai Tao, giant mangrove and the Ko Chang (Ranong) campsite (Mu Ko Ranong National Park) | 1 May-31 October | DNP 2026 table |
| Khao Ra hiking trail on Koh Phangan (Than Sadet-Ko Pha-ngan National Park) | 15 May-15 November | DNP 2026 table |
| Doi Pui peak and campsite (Doi Suthep-Pui National Park, Chiang Mai) | 1 May-30 September | DNP 2026 table |
The Similan Islands are open until 15 May, so a trip in the first half of the month is possible, weather permitting. Ang Thong Marine Park from Samui stays open: DNP’s 2026 table closes it only from 1 November to 15 December (see Ang Thong).
Holidays and events in May
The Bank of Thailand (BOT) set the 2027 holidays for banks and financial institutions in Notification 37/2569, announced on 17 August 2026:
| Date | Holiday | Source |
|---|---|---|
| Monday 3 May | Substitute day for Labour Day (Saturday 1 May) | Bank of Thailand |
| Tuesday 4 May | Coronation Day | Bank of Thailand |
| Thursday 20 May | Visakha Bucha Day | Bank of Thailand |
Festival dates move every year, and TAT usually publishes its monthly calendar shortly before the month: the May 2026 calendar came out on 29 April 2026. We found no May 2027 programme yet. For scale, TAT’s May 2026 calendar listed:
| Event | 2026 dates | Where | Source |
|---|---|---|---|
| Bun Duean Hok and Phi Khon Nam Festival (Ban Na Sao) | 2-4 May | Loei | TAT |
| Southern Talung Festival and Travel Fair | 1-10 May | Phatthalung | TAT |
| Bua Khok Ten Million Rocket Festival | 16-17 May | Surin | TAT |
Crowds and prices in May
We give no room prices: there is no official rate series, and we have not run our own dated price check yet. What MOTS publishes is hotel occupancy by province for 2025: Phuket 74.0% in May (January 86.4%), Phang Nga (Khao Lak) 72.4%, Krabi 73.4%, Surat Thani (Samui) 77.9%, Prachuap Khiri Khan (Hua Hin) 77.0%, Bangkok 70.3%, Chiang Mai 67.9%. Occupancy is not a price, so check rates for your own dates.
Should you go to Thailand in May?
Yes, if you plan around the weather: the Gulf side (Samui, Hua Hin, Pattaya) for the beach, and cities with an umbrella and a hat. Skip May if you came for the Similan Islands after 15 May or for a calm sea in front of a west-coast Phuket hotel. The month before is Thailand in April, the month after is Thailand in June, and the full picture by season is in Thailand beaches by month.
FAQ
Is May the rainy season in Thailand?
It is the start. TMD describes the southwest monsoon as blowing from May to October, and it recorded the start of the 2026 rainy season on 15 May. By the normals May rainfall runs from 96 mm at Hua Hin to 431 mm at Takua Pa near Khao Lak, and the first half of the month is usually still hot.
Where is the best weather in Thailand in May?
For the beach, the Gulf side: Hua Hin 95.5 mm on 12.3 days, Pattaya 121.1 mm on 11.0 days and Koh Samui 131.6 mm on 14.0 days by TMD normals. The Andaman coast gets 210-431 mm. Nowhere is dry in May, so plan for afternoon showers.
How hot is Thailand in May?
Average daily highs by TMD normals run from 32.6 °C at Takua Pa to 35.1 °C in Bangkok, with Chiang Mai at 34.7 °C. Nights are 23.5-26.9 °C. In May 2026 the thermometer reached 42.1 °C in Lampang on 4 May, early in the month, before the rains.
Can you swim in Phuket in May?
Often, but with care. Rawai municipality's page on Nai Harn says waves are very strong and unsuitable for swimming from May to October, and TMD reported the southwest monsoon strengthening at the end of May 2026. Swim only where lifeguards are on duty and never under a red flag.
Are the Similan Islands open in May?
Only until 15 May. The DNP park page and its 2026 closure table close Mu Ko Similan and Mu Ko Surin National Parks from 16 May to 14 October. The 2027 table was not published when we checked, so confirm with your operator.
Is Koh Samui good in May?
It is one of the better beach choices: 131.6 mm on 14.0 days at the Samui station, against 236.9 mm on Phuket, with highs of 32.9 °C. TMD has no stations on Phangan or Tao, so treat Samui as a guide. Samui's wet peak comes later, from October to December.
Which holidays fall in May 2027?
Banks close on Monday 3 May (substitute for Labour Day), Tuesday 4 May (Coronation Day) and Thursday 20 May (Visakha Bucha), per the Bank of Thailand list for 2027. With the weekend, that makes four days in a row (1-4 May).
Sources
- TMD. Climatological Data for the Period 1991-2020 (stations 48 327, 48 455, 48 461, 48 475, 48 501, 48 550, 48 561, 48 563, 48 564)
- TMD. Thailand's seasons and climate, PDF pp. 2, 4 (in Thai)
- TMD. 3-month climate prediction, May-July 2026 (issued 29 April 2026)
- TMD. Monthly Weather Summary, May 2026
- MOTS. Tourism statistics by province 2025 (xlsx)
- DNP. National park closures 2569 (2026), table dated 7 January 2026
- DNP. Mu Ko Lanta National Park (Ko Rok season)
- DNP. Mu Ko Similan National Park
- BOT. Notification 37/2569: financial institutions' holidays for 2027 (unofficial English translation)
- TAT. Important information on marine tourism during the Southwest monsoon season (24 July 2018)
- Rawai Municipality. Nai Harn Beach (in Thai)
- Karon Municipality. Karon Beach (in Thai)
- TAT. May 2026 Festivals and Events in Thailand (29 April 2026)
- PCD Regional Environment Office 14. PM2.5 risk periods by region (10 August 2026, in Thai)
Found an error or outdated fact? Tell the editors, we will fix it and update the check date.