Aargau ( AR-gow, Swiss Standard German: [ˈaːrɡaʊ] ), more formally the Canton of Aargau (German: Kanton Aargau; Romansh: Chantun Argovia; French: Canton d'Argovie; Italian: Canton Argovia), is one of the 26 cantons forming the Swiss Confederation. It is composed of eleven districts and its capital is Aarau .

Aargau is one of the most northerly cantons of Switzerland, by the lower course of the Aare River, which is why it is called Aar-gau ("Aare province"). It is one of the most densely populated regions of Switzerland.

The area of Aargau and the surrounding areas were controlled by the Helvetians, a tribe of Celts, as far back as 200 BC. It was eventually occupied by the Romans and then by the 6th century, the Franks. The Romans built a major settlement called Vindonissa, near the present location of Brugg .