The calculated distance (air line) between Donostia / San Sebastián and Biarritz is approximately 24 miles respectively 39 kilometers.
By car or train, the actual journey to Biarritz is certainly longer, as only the direct route (as the crow flies) between Donostia / San Sebastián and Biarritz has been calculated here.
Air line (approximately)
39 kilometers
21 nautical miles