Blokzijl is a small city located southwest of Steenwijk in the province of Overijssel, the Netherlands.

The city is a major tourist destination near the De Weerribben-Wieden National Park and attracts many water sports enthusiasts.

After the Siege of Steenwijk (1580–81) in the Eighty Years' War, the Dutch built a fortified lock or in local dialect zijl or siel.

This cannon was not used for defense purposes, but as a warning device during storm and high tide.

One residential building shows the height the water reached during a flood in 1825.