Map from Google - Benin

The climate in Benin is tropical, with a dry season in winter, and a rainy season, due to the African monsoon, which runs from May to September in the extreme north, and from mid-March to October in the south; along the coast, there's a pause in the rainy season in August.
The annual precipitation remains below 40 inches per annum in the extreme north, where the dry season lasts nearly seven months, while in the rest of the country it ranges between 40 and 50 in.
Here is the average precipitation of Cotonou, the main city, located on the coast.
Average precipitation - Cotonou
CotonouJanFebMarAprMayJunJulAugSepOctNovDecYear
CotonouJFMAMJJASONDY
Prec. (in)0.41.435.37.7145.72.63.94.91.60.851.6
Days1247111585884275

Here are the average temperatures of Cotonou. As we can see, on the coast it is hot all year round, and indeed the cooler months are those of summer, when the prevailing currents are those from the sea.
Average temperatures - Cotonou
CotonouJanFebMarAprMayJunJulAugSepOctNovDec
CotonouJFMAMJJASOND
Min (°F)757779797775757375757575
Max (°F)889090908884828282868888

In Benin, winter, from November to January is hot: in the north it's often torrid, with highs that can exceed 95 °F, while along the coast it's less hot, with highs around 88/90 °F, but more humid, and with a little morning cloudiness.
In the center and north, a wind from the desert blows in winter, called Harmattan, which can fill the sky with dust and make it white, creating a peculiar mist in the early hours of the day. Moreover, the nights are cool in the north, indeed sometimes even cold, so that the lowest records are about 41 °F.
Here are the average temperatures of Kandi, located in the north.
Average temperatures - Kandi
KandiJanFebMarAprMayJunJulAugSepOctNovDec
KandiJFMAMJJASOND
Min (°F)647077797773727272726463
Max (°F)889397999590868486919390

From February to May, the temperature increases further in the north, often reaching 105 °F: in the extreme north, the W National Park is particularly hot, where the temperature can touch 113 °F; even the Pendjari National Park is very hot, where, however, the rains arrive a few weeks before, bringing a bit cooler conditions.

Pendjari National Park

The first showers begin in March in the far south, and gradually move from the coast to the north. In the capital, Porto-Novo, near the eastern part of the coast, the wettest month is by far June, with more than 12 in of rain, then from mid-July to late August the rains decrease (while cloudiness remains widespread), and then they resume in September and October, but without excesses, around 5.5/6 inches per month. In the inland areas instead the wettest period goes from July to September; in these months the daytime temperature remains below 85 °F, but the relative humidity is high and the cloud cover is frequent.
Here is the average precipitation in Kandi.
Average precipitation - Kandi
KandiJanFebMarAprMayJunJulAugSepOctNovDecYear
KandiJFMAMJJASONDY
Prec. (in)00.10.41.64.15.77.910.47.71.60.1039.6
Days001381014171440071

The rains cease in early October in the extreme north, and gradually recede towards the south; on the coast they cease about a month later. With the return of the warm northeast winds, the temperature increases, while the air returns to be dry in inland areas.

The sea temperature in Benin is high throughout the year, as can be seen from the following table.
Sea temperature - Cotonou
CotonouJanFebMarAprMayJunJulAugSepOctNovDec
CotonouJFMAMJJASOND
Sea (°F)828284848482797779818482

When to go

The best time to visit Benin goes from November to January: the air is dry and the weather is usually good. Actually, it's hot during the day, especially in the north, but less than in the following period, from February to May. The rainiest and sultriest months are May and June on the coast, while they are July, August and September in the center and the north.

What to pack

In winter: in Cotonou and the coast, light clothes for the day, a light sweater for the evening, light scarf for the wind-borne sand and dust. In the north, light clothing, a sweatshirt and a light jacket for the evening, a scarf for the desert wind.
In summer: light clothing of natural fibers, a light sweater and a light raincoat for the rain showers; for women it's better to avoid shorts and miniskirts.