The calculated distance (air line) between Bilbao and Donostia / San Sebastián is approximately 48 miles respectively 77 kilometers.
By car or train, the actual journey to Donostia / San Sebastián is certainly longer, as only the direct route (as the crow flies) between Bilbao and Donostia / San Sebastián has been calculated here.
Air line (approximately)
77 kilometers
42 nautical miles