Karl Johanslussen ("Lock of Charles John") is a lock and a sluice, along the Söderström river connecting and controlling the flood discharge between Riddarfjärden, the easternmost part of Lake Mälaren, and Saltsjön, the section of the Baltic Sea reaching into central Stockholm, Sweden. The lock is 75 metres long, 10 metres wide and 3.90 metres deep. The maximum height is 3.8 metres. The lock is closed from 2016 to 2021 due to large reconstruction works in the area.