São Tomé and Príncipe sit on the equator, so temperatures stay between 22°C and 30°C year-round at sea level — cooler in the interior highlands. The rhythm is rainy versus dry, not hot versus cold.
Dry season (June–September) is the best time to visit. Trails through the rainforest are passable, the beaches are clear and calm, sea turtles nest on Príncipe between July and October, and visibility for diving is at its best. Days are warm but not stifling, and humidity drops.
Short dry spell (January–February), called gravanito, gives another window of clearer weather between the two rainy stretches. Useful if a longer trip isn't possible.
Main rainy season (October–May) brings sustained downpours, especially in the south and on Príncipe. Roads turn to mud, hiking becomes serious work, and some lodges close. Birding, however, is good — the endemic species are easier to spot when the forest is feeding well.
The Festa de São Lourenço in early August is the biggest cultural event, with parades and tchiloli dance performances. Bring insect repellent year-round; malaria is present but largely under control.