A clear, sourced, month-by-month guide to Rayong's Gulf-coast climate — the cool dry season, the hot season and the rainy monsoon, why Koh Samet stays drier than the mainland, sea and beach conditions, the best months to come, and exactly what to pack.
Rayong is warm and humid year-round, with three broad seasons: a cool, dry stretch from December to February (the best time to visit or move), a hot season from March to May peaking around Songkran in April, and a rainy southwest monsoon from roughly May to October, wettest in September. Long-term averages (1991–2021, compiled climate data) put annual rainfall at around 1,807mm, with December the driest month (≈18mm) and September the wettest (≈307mm). Because Rayong faces the sheltered Gulf of Thailand, monsoon rain tends to fall in short, heavy bursts rather than all day — and Koh Samet, partly shielded by the Chanthaburi mountain range and the Rayong peninsula, stays drier still than the mainland coast in the same months. If you want the calmest sea and the lowest rainfall — for a holiday, a Koh Samet trip, or a move — aim for the cool season.
Rayong's best and busiest window. Northeast-monsoon air brings the lowest humidity and rainfall of the year — December sees just a handful of rainy days province-wide — with warm days and comfortably cooler evenings. This is peak season for both Ban Phe/Koh Samet tourism and EEC corporate relocation activity, so book housing and ferries ahead.
Heat and humidity climb steadily, peaking around April, with Songkran (Thai New Year) celebrated in mid-April. Rainfall is still comparatively light in March before picking up noticeably by May as the monsoon takes hold. Sea temperatures are at their warmest all year; plan strenuous outdoor time for mornings and evenings.
The southwest monsoon brings Rayong's wettest stretch, peaking in September. Because Rayong faces the sheltered Gulf of Thailand rather than the open Andaman Sea, rain typically arrives as short, heavy afternoon or overnight downpours rather than continuous all-day grey, and Koh Samet in particular is further shielded by the Chanthaburi mountain range and the Rayong peninsula — the island sees noticeably less rain than the mainland coast in these same months.
Long-term (1991–2021) daytime highs, overnight lows, monthly rainfall, rainy-day counts and approximate Gulf water temperature, per compiled climate-station data. Figures are historical averages — individual years vary, and the monsoon months in particular swing from bright to stormy day to day.
| Month | High | Low | Rain | Rainy days | Sea |
|---|---|---|---|---|---|
| January | 30° | 23° | 31 mm | 5 | ≈28°C — calm & clear |
| February | 31° | 24° | 36 mm | 6 | ≈28°C — calm & clear |
| March | 31° | 25° | 88 mm | 11 | ≈29°C — warm, calm |
| April | 31° | 26° | 138 mm | 14 | ≈30°C — warmest, calm |
| May | 30° | 26° | 234 mm | 18 | ≈31°C — warm, choppier |
| June | 30° | 26° | 216 mm | 18 | ≈30°C — variable |
| July | 30° | 26° | 212 mm | 18 | ≈29°C — variable |
| August | 30° | 26° | 204 mm | 19 | ≈29°C — variable |
| September | 29° | 25° | 307 mm | 19 | ≈29°C — wettest, choppier |
| October | 30° | 24° | 260 mm | 17 | ≈29–30°C — easing |
| November | 30° | 24° | 63 mm | 7 | ≈29°C — calming |
| December | 30° | 23° | 18 mm | 3 | ≈28°C — calm & clear |
Temperatures in °C. "Rain" is the long-term monthly average total for Rayong, not a daily certainty — see the Thai Meteorological Department (TMD) for current forecasts.
For the most reliable weather, come between December and February: dry, sunny days, low humidity and calm seas for the Ban Phe–Koh Samet ferry crossing. The trade-off is peak-season demand on ferries, hotels and short-term rentals, especially around the New Year period. March to April is hot and mostly dry, good if you handle heat well and want Songkran's festival energy, with May marking the monsoon's arrival. The May–October monsoon is the value window — lower rents and hotel rates, a quieter coast and warm sea, in exchange for short daily downpours on the mainland. If Koh Samet is the priority, its partial shelter from the Chanthaburi range means it holds up better than the mainland even in the wettest months. Many EEC relocators deliberately arrive in the cool season to settle in comfortably, then experience their first rainy season once they know the area.
| Season | What to bring |
|---|---|
| Cool dry season (Dec–Feb) | Light, breathable daywear plus a light layer for breezy evenings and strong air-conditioning indoors. Sunscreen, hat and sunglasses year-round. |
| Hot season (Mar–May) | The lightest fabrics you own, high-SPF sunscreen, a refillable water bottle and a hat. Plan strenuous activity for mornings; expect to want air-con downtime midday. Bring a waterproof phone pouch if you're around for Songkran in mid-April. |
| Rainy season (May–Oct) | Everything for the heat plus a compact umbrella or packable rain jacket, quick-dry footwear and a dry bag for electronics — especially useful for the open Ban Phe pier crossing to Koh Samet. Showers pass quickly, so you rarely lose a whole day. |
December through February is Rayong's best window: the driest, least humid stretch of the year, with warm days, calm seas and the lowest rainfall — December alone averages only around 18mm of rain across roughly 3 rainy days. It's also the busiest season for the Koh Samet ferry and EEC corporate relocation activity, so book housing and boat transport ahead. Late November and early March offer a similar climate with slightly fewer crowds.
Rayong's rainy season runs roughly May to October with the southwest monsoon, peaking in September at around 307mm of rain across about 19 rainy days. Because Rayong faces the sheltered Gulf of Thailand rather than the open Andaman coast, rain tends to fall as shorter, heavier downpours rather than continuous all-day rain, with plenty of sunshine between showers. Prices are lower and the coast is quieter through these months.
Rayong is warm to hot and humid all year. Long-term averages put daytime highs around 30°C in the cool season, climbing to roughly 31°C around the April peak, while overnight lows rarely drop below the low-to-mid 20s°C even in the coolest months. High humidity — regularly 80%+ during the monsoon — makes it feel hotter than the thermometer suggests, especially from May onward.
Yes — Koh Samet is genuinely drier than the mainland coast during the same months. The island sits partly in the rain shadow of the Chanthaburi mountain range and the Rayong peninsula, so southwest-monsoon rain more often passes over or around it than lands directly on it. You'll still get more cloud and the occasional afternoon shower May–October, but full washout days are the exception rather than the rule, even during the September–October mainland peak.
Pack light, breathable clothing whatever the month, plus high-SPF sunscreen, a hat and sunglasses. Add a light layer for cool-season evenings and strong air-conditioning indoors, and for the rainy season (May–October) bring a compact umbrella or packable rain jacket, quick-dry shoes and a dry bag for electronics — handy for the open Ban Phe pier crossing to Koh Samet. If you're around for Songkran in mid-April, a waterproof phone pouch is worth having.
It's a separate but related consideration. Rayong hosts the Map Ta Phut industrial estate, and February–April — the tail of the dry season — is also the region's haziest window due to regional crop burning stacking on the local industrial baseline, mainly affecting Rayong city and the industrial corridor rather than Ban Chang, Ban Phe or the Koh Samet side. See the Rayong air quality guide for the month-by-month detail and which areas stay clearest.
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Whether you come for the cool-season sunshine or the value of the green season, match the right area to how you want to live, then browse condos, villas and houses there.
General climate information based on long-term averages; actual weather varies year to year — check a current TMD forecast before you travel. Hero photo by Connor Scott McManus on Pexels.