The Urdaibai estuary is a natural region and a Biosphere Reserve of Biscay, Basque Country, Spain. It is also referred as Mundaka or Gernika estuary.

The Basque Country, also called the Basque Autonomous Community, is an autonomous community in northern Spain. It includes the Basque provinces of Álava, Biscay, and Gipuzkoa. It also surrounds an enclave called Treviño, which belongs to the neighboring autonomous community of Castile and León.