The Außenalster (meaning "outer Alster") is one of two artificial lakes within the city limits of Hamburg, Germany, which are formed by the river Alster (the other being the Binnenalster). The size of the Außenalster is 1.6 km².

The phrase "outer" refers to the old city walls of Hamburg. The Außenalster was the part of the lake that was "outside" the city walls. Today the old city walls do not exist, instead two car and train bridges, the Lombardbrücke and the Kennedybrücke , span the river.

The Außenalster is used by the inhabitants of Hamburg for recreational purposes such as sailing and rowing. Almost all banks of the Außenalster are public. The banks vary from a small strip of green to large public parks (e.g. the Alstervorland ). It is very popular to jog around the lake (approx. 7 km).