Ecuador straddles the equator, which means temperatures don't swing dramatically between seasons the way they do at higher latitudes. What matters more is rainfall — and the patterns differ markedly between the highlands, the coast, and the Amazon.
The Andes (Sierra). June to September is the dry season and the best time for hiking, volcano climbing, and clear mountain views. Days are sunny and crisp; nights are cold. The wet season runs roughly October to May, bringing afternoon rain and frequent cloud. Travel is perfectly doable in the wet season — the landscape turns vivid green and the highland markets continue regardless — but mountain views can be obscured for days at a time.
The Pacific Coast. The warmest, most swimmable conditions run from December to April. Yes, this is technically the wet season, but on the coast rain usually means short afternoon downpours rather than all-day grey. May to November is drier but notably cooler, with a persistent mist called garúa common on the southern coast.
The Amazon. Rain falls year-round. The slightly drier months are August to November, which makes jungle trails more walkable, but the Amazon is fundamentally wet — embrace it. River levels are highest from March to July, which can be an advantage for boat travel and reaching remote oxbow lakes.
The Galápagos. The islands have two seasons. December to May is warmer (water temperatures around 24°C), with calmer seas and more rain. June to November is cooler and drier, with stronger currents — better for diving and snorkelling with cold-water species like hammerheads and penguins. Wildlife viewing is excellent year-round; different species peak in different months.
Peak tourist season is mid-June through August and the Christmas–New Year period. Prices rise and popular lodges fill quickly. If you're going to the Galápagos, book well ahead regardless of season.