The Río San Juan region is wet year-round, but the driest months are February to April. From May to November rainfall is heavy and the river can rise significantly, sometimes making travel more difficult and hiking conditions in the reserve demanding. December and January offer a reasonable compromise — some rain but manageable.
The upside of visiting in wet season is that the forest is at its most intense and wildlife-spotting can be exceptional: the animals are more active and less concentrated around drying water sources. Mosquitoes are present throughout the year — bring repellent, wear long sleeves at dawn and dusk, and check whether malaria prophylaxis is recommended for your itinerary.