Ninety kilometres east to a coast most people fly over on their way to Koh Samui โ and the mainland beach nearest the island.
Find a room before you go.
Khanom is about ninety kilometres east of Surat Thani, on the mainland coast opposite Koh Samui. Minivans run from Talad Kaset 2 bus station in Surat Thani roughly every forty-five minutes between seven in the morning and six in the evening, take a little over an hour and cost around a hundred baht.

Khanom has no railway station of its own. Surat Thani is the nearest at ninety kilometres, and Nakhon Si Thammarat is the other option to the south. So the pattern for most people is train to Surat Thani, then van.
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The standard way and the most frequent. Buy the ticket at Talad Kaset 2 and say you want Khanom, not Nakhon Si Thammarat, or you will sail past the turn-off.
Fewer departures than the van and a little slower, but more room and a proper luggage hold.
East on Highway 401 then north up the coast. Worth it if you want to see Khanom and Sichon in the same day, which is easily done.
Step-by-step transport guides for this stretch of the Gulf coast.