Busturialdea, also named Busturialde - Urdaibai is a comarca of the province of Biscay, in the Basque Country, Spain. It is the heir of "Busturia", one of the original merindades that used to compose the province of Biscay – this should not be confused with Busturia, a municipality located in this region. Busturialdea is a diminished portion of the same original subregion and has two capital cities; Bermeo and Gernika-Lumo. It is one of the seven comarcas that compose the province of Biscay.

In Spain, a comarca is either a traditional territorial division without any formal basis, or a group of municipalities, legally defined by an autonomous community for the purpose of providing common local government services. In English, a comarca is equivalent to a district, county, area or zone.