Brunei is equatorial — hot and humid year-round with no real cool season. Temperatures in Bandar Seri Begawan stay in the low-to-mid 30s Celsius during the day and rarely drop below 23°C at night. What changes between seasons is the rain.
The driest months are February and March, making them the most comfortable time to visit. January through May is broadly the dry season, with lower humidity and more reliable sunshine. February is also when Brunei celebrates National Day on the 23rd — a major event with military parades and public festivities.
The wettest period runs from October through January, when heavy downpours arrive most afternoons and the Temburong rivers can run high. Rain is usually intense and short-lived rather than all-day grey, so a wet season visit is entirely manageable, particularly if your main interest is the mosques and the capital.
If you plan to visit Ulu Temburong National Park, the dry season is preferable — river levels are more predictable, the canopy walk is more accessible, and the forest trails are less muddy. That said, the park is operated by guided tour and professionals manage the logistics regardless of season.
July to August falls in a warmer, more humid shoulder season. The timing coincides with school holidays across the region, so expect more domestic and regional visitors.