The Södertälje Canal is a canal connecting the lake Mälaren with the Baltic Sea, at the city of Södertälje. It is 5,2 km long, and it has one lock. The size of this lock is the largest in Scandinavia by allowed ship size. The lock is 135 m long and 17 m wide. The water level difference between Mälaren and the Baltic Sea is not large, on average 0,6 m (2 ft). This is the main shipping route into the lake Mälaren.
In the Viking age the passage was navigable by boats, but the Post-glacial rebound made it unusable. In the year 1819 a lock was finished, so boats could use the canal. In 1924 a much larger lock (remains in use today) was built and the canal was widened and deepened.