Twice-weekly flights from London City airport will operate from July through September 2020.
The town in the Bay of Biscay has a traditional old town and access to sandy beaches.
It is also renowned for its restaurants, and being the town with the highest concentration of bars in the world.
Rachel Riley, BA head of commercial at London City, said: “The launch of this new route from London City will prove very popular with our leisure customers looking for new places to explore.
“It will be the first time San Sebastian is served from London City, and we are excited to offer our customers another easy getaway leaving from the heart of London.”
London City aviation director Peter Downes added the route would serve as a gateway to other destinations in the Basque region, as well as Cantabria, Pamplona and the Pyrenees.
“It’s sure to be a favourite for summer 2020 getaways,” he added.
San Sebastian fares lead in from £50 each way, hand baggage only.
Flights depart London City at 9.10am on Fridays, arriving 12.10pm, and 9.15am on Mondays, also arriving 12.10pm.
BA is shaking up its schedule at London City for summer 2020, adding flights to Berlin, Florence, Glasgow, Manchester and Nice while reducing frequency to Geneva, Ibiza and Rotterdam.
