Hjälmaren (Swedish pronunciation: [ˈjɛ̂lːmarɛn] or [ˈjɛ̂lːmaɳ]), also spelled Jälmaren, is Sweden's fourth largest lake. It is situated in the Central Swedish lowland and drains through Eskilstunaån into the adjacent Lake Mälaren, which in turn drains into the Baltic Sea, west of Stockholm. It is connected by waterway with Stockholm by the 13 kilometres long Hjälmare kanal.
It is bounded by the provinces of Södermanland, Närke, and Västmanland.