French Guiana has a tropical climate with two wet seasons and two dry seasons. The main dry season runs from August to November, which is generally the best time to visit: roads are passable, boat crossings to the Îles du Salut are more reliable, and the humidity is at its least oppressive. The secondary dry season in February and March is also workable.
The main wet season (May to July) brings heavy rains and can make overland travel difficult; the secondary wet season (December to January) is shorter and less intense. Rain here is genuinely tropical — brief, torrential, and usually over within an hour. Outside the dry seasons you won't be rained out for days at a time, but you should expect daily downpours.
If you're timing a visit around a rocket launch from the CSG, check the Arianespace launch schedule in advance — launches happen year-round and are not affected by the wet season (though launches are occasionally delayed for unrelated technical reasons). Getting a confirmed launch date before booking travel is difficult since schedules shift, but watching a launch is spectacular enough that it's worth checking. The viewing area is open to the public with tickets available through tour operators in Kourou.
October to February is sea turtle nesting season on the beaches, adding a wildlife dimension to the coastal walks.