Train from Santiago De Compstela to Paris

The French and Spanish trains don’t cross borders on the Atlantic side. I think you’d have to take a Spanish train and walk across the border. I’m not sure if the TGV goes all the way to HEd aye, or you might have to take a local train to Bayonne/Biarritz for the train to Paris.

Check https://www.sncf-connect.com/ but the train schedule isn’t available yet for Sun. Jun 4. I took an overnight train from San Sebastian to Blois and had a transfer in Vitoria/Gastiez (Paris was the end of line). You can also check https://www.renfe.com/es/en about 60 days out or less.
I highly recommend the overnight train if there is one and rent a sleeper car because it’s a long ways. I don’t see a nonstop flight from San Sebastian to Paris on Jun 4 but Bilbao does. The further out you book on a low cost carrier the cheaper it is.

If the international border causes too much delay in train travel, Vueling (Iberia subsidiary) and Hop (Air France subsidiary) offer non-stop flights from Bilbao to Paris on June 3 and June 4. There are no non-stop flights from the smaller San Sebastian airport on either date

From San Sebastian you can take the Euskotren to Hendaye. It runs about every thirty minutes. In Hendaye you can get the TGV to Paris. Generally the trip is less than 5hours.

Just to add my voice to what dlindstrom said.

The Euskotren is a good local train service, think of something similar to the metro. It isn't fast, but it is clean and it's only a short distance from Donastia St Sebastian anyway. 40 minutes.

At Hendaye it's a one minute walk from the Euskotren station to the SNCF station

We did it before the plague hit, with the difference that we then caught a bus at the station at Hendaye. We wrote about it here: https://daysontheclaise.blogspot.com/2019/10/catching-train-from-spain.html?m=1

Due to bureaucracy the TGVs don't cross this border anymore. But there are direct TGVs from Hendaye, just across the border, to Paris. To get there from Donostia/San Sebastián, take the local Euskotren to Hendaye.

Flying from Bilbao would mean a longer and more expensive trip, as well as more faff.

From San Sebastian you can take the Euskotren to Hendaye. It runs about every thirty minutes. In Hendaye you can get the TGV to Paris. Generally the trip is less than 5hours.

I forgot about this, thank you.

The title mentions San Sebastian, the text Santiago De Compostela. From where is it that you want to travel from Paris? These places are at opposite ends of the country...

Thanks so much for pointing that out. I'll edit it.

Santiago de Compostela is a long way from Paris. The obvious choice would be to fly.

There used to be a night train from Santiago de Compostela (and other places in Galicia) to Barcelona, where you could then connect with the TGV to France. I did travel that route once, a long time ago and it was nice. But that night train no longer runs.

The route would be Santiago de Compostela - Madrid - Barcelona - Paris. Currently not possible to do this in one day, so you would need to break it either in Madrid or Barcelona. But check the schedules again in a few months. RENFE has the habit of scheduling trains at short notice...

Another possible option would be to train or bus from Santiago de Compostela to Porto and then fly from there to Paris.

Vueling and Transavia have non-stops from SdeC to Paris on June 4, but not on June 3. Vueling's 1:50 PM departure is scheduled into CDG at 2:55 PM. Connecting flights provide more options.

There's also an airport in A Coruna, not too far from SdeC, but it has no non-stops to Paris for your travel dates.