Plentzia is located in the Plencia - Mungia region, and is bordered by the final stretch and mouth of the Butrón river (or Ría de Plentzia). It occupies a land area of 11.17 square kilometres.
It stretches over a series of hills, rising gradually in height to Zabala-Ondo (213 m) and Kukutxa (109 m). Structurally, this area forms part of the "Bizkaia Sinclinorium", with a predominance of Eocenic materials.
The river forms a series of meanders, in one of which lies the urban nucleus of Plentzia.
Three geographically scattered districts make up the municipal area-Saratxaga, Isuskiza and the urban centre.
The urban centre consists of three well differentiated nuclei: the historical centre (the original nucleus of the town), Txipios and the "Ensanche" (or Extension), mostly consisting of buildings that reflect the recent urban growth of the town.