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.
Urdaibai is located on the Bay of Biscay coast, in the north of the Iberian Peninsula. Urdaibai covers an area of 220 km² with some 45,000 inhabitants, most of them concentrated in the towns of Bermeo and Gernika. The territory is characterized by a hydrographic basin of small streams that merge in a great salt marsh surrounded by high sheer cliffs. The surrounding countryside is occupied by meadowland, oak groves, leafy woods and specially by plantations of fast-growing conifers. The coastal landscape is covered with Cantabrian woods of Holm Oak and Strawberry Trees.
In Urdaibai have been described 615 species of vascular plants and 318 species of vertebrates, 245 of which are birds.
The area contains remnants of the Prehistoric, Roman and the Middle Ages. Particularly notable is the Santimamiñe cave, which has yielded archeological artifacts from the Mousterian to the Iron Age.
The economy is based on metallurgy, fishing, farming and forestry. An important tourist sector is based on the quality of the local beaches.