Map from Google - Sao_Tomé_and_Principe

In the islands of São Tomé and Príncipe, located in the Gulf of Guinea and in the vicinity of the Equator, the climate is tropical, hot and humid throughout the year, with maximum temperatures around 30 °C (86 °F) from January to April, and a slightly cooler period from June to August. Both islands are covered with impenetrable forests, and in inland areas there are hills wrapped by mists.
Here are the average temperatures of Sao Tomé.
Average temperatures - Sao Tomé
Sao ToméJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)222222222120202020212121
Max (°C)292930292927272727272828
Min (°F)727272727068686868707070
Max (°F)848486848481818181818282

The rains are abundant on the island of Príncipe, around 2,000 millimetres (79 inches) per year in the main town of Santo António, and also in the south of São Tomé, where even 5,000 mm (195 in) per year fall in the south-facing slopes, while they are scarcer in the north of São Tomé, where the capital is found, because it's located in the "rain shadow" of the prevailing south-western currents, which discharge rainfall on the southern slopes of Pico de São Tomé, the highest mountain in the country with its 2,024 metres (6,640 feet). However, even in São Tomé the vegetation is lush, due to the high humidity and the constant presence of clouds. In São Tomé, about 900 mm (35.5 in) of rain per year fall.
Here is the average precipitation.
Average precipitation - Sao Tomé
Sao ToméJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)8085130120115190117110100110887

In both islands, there is a dry season, a little shorter in Príncipe (mid-June to mid-September), and longer in São Tomé (June to September), but even in these months, the sky is often cloudy. On the contrary, the rainiest months are April and May, October and November, because of the two passages of the sun at its zenith.

São Tomé

The sea temperature is high enough for swimming all year round.
Sea temperature - Sao Tomé
Sao ToméJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)282929292826262626272728
Sea (°F)828484848279797979818182

When to go

The best time to visit São Tomé runs from June to September, while in Príncipe it runs from mid-June to mid-September, being the least rainy months, when it rarely rains. However, it should be remembered that the sky is often cloudy even in this season.

What to pack

Throughout the year, tropics-friendly, loose fitting clothing, light shirts and pants of natural fibres (cotton, linen) or synthetics that breathe, possibly a light sweater for the evening; light raincoat or umbrella for the rain. From June to September, you can add a light jacket for the evening.
To climb Pico de São Tomé, sweater, jacket, raincoat, hiking shoes.