Blokzijl is a city located southwest of Steenwijk in the province of Overijssel, the Netherlands.
Blokzijl grew in the 1580s. During the siege of Steenwijk, the Dutch built there a fortified lock or in local dialect zijl or siel. It received city rights in 1672. Until 1973, it was a separate municipality; it is now part of Steenwijkerland.