Mälaren (UK: MEL-ər-en, US: MAY-lar-ən, Swedish: [ˈmɛ̂ːlarɛn] (listen) or [ˈmɛ̂ːlaɳ]), historically referred to as Lake Malar in English, is the third-largest freshwater lake in Sweden (after Vänern and Vättern). Its area is 1,140 km² and its greatest depth is 64 m. Mälaren spans 120 kilometers from east to west. The lake drains, from south-west to north-east, into the Baltic Sea through its natural outlets Norrström and Söderström (as it flows around Stadsholmen island) and through the artificial Södertälje Canal and Hammarbyleden waterway. 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 and Västmanland. The two largest islands in Mälaren are Selaön (91 km²) and Svartsjölandet (79 km²). Mälaren is low-lying and mostly relatively shallow.

Being a quite narrow and shallow lake, Mälaren has bridge crossings between Eskilstuna and Västerås with two crossings on the western end at Kvicksund and three separate bridges between Strängnäs and Enköping in the central part of the lake. On the eastern end, the entirety of Ekerö Municipality is set on islands within Mälaren. That urban area also has a bridge connection to the mainland in Stockholm along with bridges between various islands in the municipality.

The Viking Age settlements Birka on the island of Björkö and Hovgården on the neighbouring island Adelsö have been a UNESCO World Heritage Site since 1993, as has Drottningholm Palace on the island of Lovön. The barrow of Björn Ironside is on the island of Munsö, within the lake.