Bus from Bilbao to San Sebastián

What makes you believe train is not feasible?

Euskotren runs that route at least once per hour, they are dirt cheap, and they have multiple stations in each town, so you can pick one closest to your accommodation.

That train journey is really very feasible. Is there some particular issue on the day you are travelling?

Train is run by a public company called Euskotren, narrow gauge trains designed for our narrow valleys and mountainous/hilly areas. The ride from Bilbao to Donostia/San Sebastian takes 2.5h because it stops in all towns on the way, but it´s very scenic and absolutely doable. Buses are frequent, clean, cheap, comfortable, modern and fast, 1.15h to get to D-SS, two companies, Alsa and Pesa, identically good.

When I look for the train from San Sebastian to Bilbao, I'm only finding trips that take 6 hours or more. Where can I find ones that are 2.5 hours?

I’m having same problem- Renfi is 6 hours. I’ll look into this smaller train. Thanks.

RENFE is not the only train operator in Spain (there are many), it would seem very odd to cover 100km/60miles in 6 hours on a train ride. https://www.euskotren.eus/en/tren/schedules for the local Basque narrow gauge train operator and schedules. Again, it´s a train designed for linking small towns, not a long distance train.

The Euskotren trip has very nice scenery. It's a perfectly pleasant ride if you aren't in a hurry (in which case, take the bus instead). But be prepared for train cars that look like subway cars; I think the seats are plastic, etc. Perfectly serviceable, but not especially comfortable.

We were just there , bus is easy and faster than the train.

We just booked the bus from Bilbao to San Sebastian, through an app called Omio. I believe the bus is Alsa.

We are taking the bus from Bordeaux to Bilbao, then from Bilbao to San Sebastian, then from San Sebastian back to Bordeaux. We took the Alsa bus once from Granada to Seville and thought it was quite comfortable.

Can also book directly through Alsa Bus

It would be interesting to see what the OP decided to do.

We took the train (Euskotren) in 2018 and enjoyed the ride.
Very scenic.