Illgau is a municipality in Schwyz District in the canton of Schwyz in Switzerland. Illgau is first mentioned in 1370 as Ilgoe. Illgau has an area, as of 2006, of 10.9 km². Of this area, 62% is used for agricultural purposes, while 31.1% is forested. Of the rest of the land, 2.9% is settled and the remainder is non-productive.