is the third largest lake in Sweden, after Lakes Vänern and Vättern. Its area is 1140 km² and its greatest depth is 64 m. The lake drains, from west to east, into the Baltic Sea at Stockholm through Södertälje kanal, Norrström, Slussen and Hammarbyslussen. The easternmost bay of Mälaren, in central Stockholm, is called Riddarfjärden. The lake is located in Svealand and bounded by the provinces of Uppland, Södermanland, Närke, and Västmanland. The two largest islands in Mälaren are Selaön (91 km²) and Svartsjölandet (79 km²).

The presence of the Zebra mussel is causing serious problems in Mälaren and is considered an invasive species.

Birka (older name for Björkö) and Hovgården (on the neighbouring island Adelsö) has been a UNESCO World Heritage Site since 1993.

According to Norse mythology, the lake was created by the goddess Gefjun when she tricked Gylfi, the Swedish king of Gylfaginning. The land thusly removed by Gefjun was transported to Denmark, becoming the island of Zealand.