Bangladesh is small enough that people assume one weather answer covers it. It mostly does not. The coast, the northeastern tea hills, and the southwestern mangroves each have a different best window, and two of them are actively worse in the month that suits the third. If your trip is built around one region, plan around that region rather than around the country.
The national baseline
Four seasons, roughly. Winter (November–February) is cool, dry and clear — peak season, and busy with it. Spring (March–May) brings rising heat and haze, still comfortable early on. Monsoon (June–September) brings heavy rain, high humidity and travel disruption from flooding. Autumn (October) is the quiet shoulder: rains easing, landscape still lush, crowds not yet back.
If you want one dependable answer for a first, broad trip taking in several regions, it is late November to early February. Everything below is why you might reasonably ignore that.
Cox's Bazar and the coast: November to March
The beach has the least ambiguous season of the three. Cox's Bazar is at its best from November through March: daytime temperatures around 20–30°C, calm seas, minimal rain.
April to October is monsoon on this coast, and the objection is not merely that it rains. The sea gets genuinely rough, which makes swimming riskier rather than just less pleasant, and boat crossings to Saint Martin's Island are curtailed or suspended for much of it. If the island is the point of your trip, this is a hard constraint, not a preference.
The same window suits Kuakata on the southern coast, where the draw is seeing both sunrise and sunset over the water and neither works well under monsoon cloud.
Sylhet and Sreemangal: it depends what you came for
The northeast is where the single-answer approach breaks down completely, because the region has two distinct attractions with opposite seasons.
For the tea gardens around Sreemangal, the pleasant window is the cooler, drier months and the early part of spring, before the heat builds — good light, comfortable walking, hillsides fully green. Plucking activity across the estates is at its most visible outside the depths of winter dormancy, so late spring and early autumn reward anyone who actually wants to see the gardens working rather than just photograph them.
For the swamp forests and waterfalls around Sylhet, monsoon is not a compromise — it is the entire point. Ratargul swamp forest is only properly flooded, and only navigable by boat through the submerged canopy, during and just after the heavy rains. The waterfalls near Jaflong likewise only run properly then. Visit Ratargul in February and you are walking beside a forest instead of paddling through it.
October is the compromise that serves both reasonably: rains easing, water levels still high, landscape at its greenest.
The Sundarbans: a narrow window
The Sundarbans has the tightest season of the three, and it is worth building the rest of your trip around rather than fitting in.
Boat safaris run best from roughly November to February. The practical reasons stack up: calm water in the channels, cooler days aboard an unairconditioned boat, far fewer mosquitoes, and better wildlife visibility as animals come to the waterline in the cooler, drier conditions. Monsoon here means rough water, heavy insect pressure, and cyclone risk on an exposed delta — several operators simply do not run.
Because you are booking a multi-day boat package rather than travelling independently, availability in that window tightens early. This is the one destination where booking well ahead genuinely matters.
The hill tracts, and a note on cold
Bandarban and Rangamati follow the winter pattern — dry ground, clear views, safer hill roads. Monsoon brings landslides and washed-out access, which is a more serious constraint here than elsewhere.
Worth knowing that “dry season” is not uniformly warm. December and January mornings in the north and in the hills get genuinely cold, and winter fog regularly delays domestic flights and ferries. Build slack into tight winter itineraries.
Dhaka and the archaeological north
Dhaka is a year-round city in the sense that nothing closes, but it is markedly more pleasant to walk in winter. Old Dhaka on foot in May heat and humidity is genuinely punishing, and the same lanes in January are one of the best days out in the country. The trade-off is air quality, which deteriorates sharply through the dry months.
The northern sites — Mahasthangarh near Bogura and the Somapura Mahavihara at Paharpur near Naogaon — follow the same logic more sharply still, because you are outdoors on exposed ground with little shade. Winter, and early in the day.
Festivals worth planning around
Two dates pull in opposite directions. Pohela Boishakh, the Bengali New Year in mid-April, is the country at its most exuberant and one of the genuine reasons to accept the heat of that month. Eid, whose date moves each year against the Gregorian calendar, empties the cities and fills every bus, train and flight in the country as people travel home — extraordinary to witness, close to impossible to travel through if you have not booked well ahead.
So when should you go?
Beach or mangroves as the centrepiece: November to February, no real argument. Swamp forests and waterfalls: monsoon, and accept the disruption. Tea gardens: the shoulders, spring or October. A broad first trip covering several regions: late November to early February, and save Ratargul for a second visit.
Month-by-month weather, festival dates worth planning around, and health notes are on the Safety & Best Time to Visit guide. For what to put in the bag once you have picked a month, see the seasonal packing list.