Seria lies on the coastal highway between Bandar Seri Begawan and the Malaysian border, about 70 km southwest of the capital. Buses from BSB run regularly and take around two hours; shared taxis cover the same route in less time and are a reasonable option if you are in a group. The road is straightforward and well-maintained.
If you are continuing to Malaysia, Kuala Belait is about 30 minutes further west by road. From Kuala Belait there are ferries across the river into Sarawak, making Seria a natural stopping point on a Brunei-to-Malaysia overland route. Most travellers heading that way pass through Seria without stopping; a half-day is enough to see the main sights if you want to break the journey.