Purmer is a polder and reclaimed lake in the Netherlands province of North Holland, located between the towns of Purmerend and Edam-Volendam.

Hydraulic engineer Jan Adriaanszoon Leeghwater also had stakes in the reclamation, although he was not directly involved in the project itself.

The original lake of Purmer formed part of a small number of landlocked minor seas located in North Holland.

All these lakes were directly connected to open sea, so salt water could flow in and tidal movements occurred.

The Purmer's being directly connected to open sea resulted in large-scale shoreline erosion due to wave dynamics and water currents.