Lake Hallwil, Lake in Aargau and Lucerne, Switzerland.

Lake Hallwil is 8.4 km long, up to 1.5 km wide and 47 m deep, covering 10.3 km², five sixths in Aargau and one sixth in Lucerne. The Aabach is its main inflow and outflow. Reeds line parts of the shore, small boats cross the water, and a snow-capped range rises beyond the far bank.

The lake formed during the Würm glaciation when a terminal moraine dammed the valley. Around 1300 it bore several names, including Halwiler se and lacus de Seingen, before the name of the Hallwyl bailiffs prevailed; they held fishing and shipping rights and their court met at Birrwil, Beinwil am See, Mosen and Seengen. The lake became publicly accessible only in 1859.

The lake's name comes from the lords of Hallwyl, who held fishing and shipping rights and lower jurisdiction over the water; their court sat on the shore at Birrwil, Beinwil am See, Mosen and Seengen, and from 1429 half of the fines went to the fishermen. Their moated castle of Hallwyl stands on an island in the Aabach about a kilometre north of the lake, not on the lake itself.

A 22 km footpath circles the whole lake, but cycling is not allowed on it. The Seetalbahn (Lenzburg–Lucerne) runs along the entire western shore, while the eastern shore is served by bus lines. Boats of the Hallwilersee shipping company operate in summer, and the lake can freeze over in cold winters.

On 17 September 1938, Sir Malcolm Campbell drove his own motor racing boat Blue Bird K3 to a world water speed record of 210.66 km/h on Lake Hallwil. The lake had been open to the public only since 1859, and the Seetalbahn had run along the western shore since 1883.