Although it's one of the newest museums in the Basque Country, the San Telmo Museoa has actually been around since the 1920s. It was closed for many years, but after major renovation work it reopened in 2011. The displays range from historical artifacts to the squiggly lines of modern art, with all pieces reflecting Basque culture and society.
Labelling is in Spanish and Basque, but there are free audio guides available in other languages. If you don't speak Spanish, take one of these audio guides as otherwise the connections between the collection's pieces will seem vague.