Bygland is Aust-Agder's second largest municipality in area. The majority of the area of the municipality lies in the Setesdalsheiene mountains, at an elevation of over above sea level. The municipality stretches from the small town Byglandsfjord in the south to the farms at Langeid in the north. Lake Byglandsfjorden is long and lies on the river Otra. Other lakes include the Åraksfjorden, Gyvatn, Hovatn, Kvifjorden, Longerakvatnet, Straumsfjorden, and Topsæ. The rivers Otra and Topdalsfjorden both run through Bygland on their way south. The Reiårsfossen waterfall is one of many waterfalls in the municipality.