The gateway to Ko Samet is Ban Phe pier in Rayong province. Ferries run from there to three piers on the island: Na Dan (25 min), Ao Prao (30 min), and Wong Duan (45 min). Services run daily from around 07:00 to 17:00. If the schedule does not suit, speedboats are available for private hire at Ban Phe for around 1,000 baht per trip.
By bus from Bangkok: Buses to Ban Phe leave from Ekkamai (Eastern) Bus Terminal. The journey takes about five hours and costs around 100–120 baht. Services run roughly every 40 minutes from 07:00 to 20:30. From Ban Phe it is a short walk or motorbike taxi ride to the ferry pier.
By taxi from Bangkok: A metered taxi or ride-share from central Bangkok to Ban Phe takes about three hours and costs 1,600–2,000 baht. Useful if you are travelling with luggage or a group.
Note that Ko Samet sits inside a national park — you will be asked to pay a national park entrance fee (currently 200 baht for foreigners) when you arrive on the island.