The best time to visit São Tomé is June to September, during the dry 'Gravana' season, when rainfall is low, seas are calm for diving and island-hopping, and hiking trails in the Obô are passable without wading through mud.
June–September (Dry / Gravana): The coolest and driest period, with temperatures around 24–27°C in the capital. Ideal for hiking, beach time, and diving. Accommodation books out faster in August — reserve early.
October–January (Short rains / Gravanito): A brief wetter spell, though the north of the island remains drier than the south. The landscape is brilliantly green, birdlife is active, and rates are lower. A good shoulder-season option for budget travellers.
February–May (Long rains): The south and interior receive the heaviest rainfall, and Pico de São Tomé can be clogged in cloud for days. Beaches in the north stay sunnier, and the island's waterfalls are spectacular. Least crowded and lowest prices.
Year-round note: Because São Tomé sits on the equator, temperatures are stable throughout the year (24–30°C at sea level). Rain is the main variable, not heat.