Map from Google - Equatorial_Guinea

In Equatorial Guinea, the climate is hot and humid all year round.
The temperatures are higher from January to May, when they hover around 30/32 °C (86/90 °F) during the day, while they are lower from July to September, when they drop to around 27/28 °C (81/82 °F) in the mainland part, and remain slightly higher in the islands.
Here are the average temperatures of the capital Malabo, located in the island of Bioko.
Average temperatures - Malabo
MalaboJanFebMarAprMayJunJulAugSepOctNovDec
MalaboJFMAMJJASOND
Min (°C)192121212221212121212221
Max (°C)313231323129292930303031
Min (°F)667070707270707070707270
Max (°F)889088908884848486868688

The rains are plentiful, but the rainfall pattern in the mainland is the opposite to that of the islands.
In the continental part, known as Rio Muni, precipitation is above 2,000 millimetres (80 inches) per year; the rains occur from September to May, while there's a dry season from June to August; in the north (see Bata) the rains decrease slightly from December to February, while in the south (see Cogo) it rains a lot even in this period. The amount of sunshine is never good, and it is even scarcer in the driest months.
Here is the average precipitation in Bata.
Average precipitation - Bata
BataJanFebMarAprMayJunJulAugSepOctNovDecYear
BataJFMAMJJASONDY
Prec. (mm)2202902753952558055751253704302452815
Prec.(in)8.711.410.815.6103.12.234.914.616.99.6110.8
Days141517191543513222217166

In the island of Bioko, precipitation remains below 2,000 mm (80 in) per year in Malabo, while it exceeds 3,000 mm (120 in) per year in the rest of the island; here the dry season runs from December to February, although it's more pronounced in the northern part, where the capital Malabo is located, while the rainiest period goes from May to October.
Here is the average precipitation in Malabo.
Average precipitation - Malabo
MalaboJanFebMarAprMayJunJulAugSepOctNovDecYear
MalaboJFMAMJJASONDY
Prec. (mm)306510518024028019017024526590401900
Prec.(in)1.22.64.17.19.4117.56.79.610.43.51.674.8
Days348912131210141383109

In the winter months, a hot dry wind from the mainland can blow. Here, too, the sky is often cloudy, especially from June to August. Malabo is one of the least sunny capitals in the world, being usually cloudy, and also foggy. It follows that the vegetation is lush.
In Bioko, at no great distance from Malabo, there is the highest peak of Equatorial Guinea, Pico Basilé, 3,011 metres (9,879 feet) high. The slopes of the volcano are very rainy, but they protect the capital, located on the north coast of the island, in the period from July to October, when it rains more than in the capital also in the eastern and southern coasts of the island.
In southern island of Annobón, located southwest of the state of São Tomé and Príncipe, the rainfall pattern is similar to that of the mainland, with a dry season from June to September. Here, too, the sky is often cloudy.

Malabo

The sea in Equatorial Guinea is warm all year round, as you can see in the following table.
Sea temperature - Malabo
MalaboJanFebMarAprMayJunJulAugSepOctNovDec
MalaboJFMAMJJASOND
Sea (°C)282929292928272627272828
Sea (°F)828484848482817981818282

When to go

The best time to visit the continental part of Equatorial Guinea (Rio Muni) goes from June to August, in the island of Annobón runs from June to September, while in the island of Bioko is from December to February.

What to pack

For the islands of Bioko and Annobón, tropics-friendly, loose fitting clothing, possibly a light sweatshirt for the evening; during the rainy season, umbrella or light raincoat. To climb Pico Basilé, sweater, jacket, hiking boots.
For Rio Muni, tropics-friendly, loose fitting clothing; during the rainy season, umbrella or light raincoat, possibly a light sweatshirt for the evening; for the hilly inland areas, where the national parks are found, you can add a sweatshirt and a light jacket.