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Agusan del Sur is an elongated basin formation with mountain ranges in the eastern and western sides forming a valley, which occupies the central longitudinal section of the land. The Agusan River, which flows from Compostela Valley in the south towards Agusan del Norte in the north, runs almost in the middle of the valley and empties at Butuan Bay. The river has twelve tributaries: Wawa, Gibong and Simulao Rivers in the eastern side and Ojot, Pusilao, Kasilayan, Libang, Maasam, Adgawan, Cawayan, Umayam and Ihaon Rivers in the western side. These tributaries are fed by streams and creeks. The southern half of the province from the municipality of Veruela is an area filled with swamps and lakes, the biggest of which is Talacogon Lake.

The province annually celebrates its Naliyagan Festival, a celebration of the foundation anniversary of Agusan del Sur which is a week-long socio-cultural and arts exposition of indigenous tribes in the entire Agusan valley and displaying their skills and gracefulness in dancing as part of their ritual activities. It is being held in the Naliyagan Plaza at the Agusan del Sur Capitol Grounds in the municipality of Prosperidad. Naliyagan is a Manobo word which means "the chosen one" or "the most loved one". Agusan Marsh, Bega Falls, Gibon River, Binaba Falls, Tugonan Falls is the tourist attraction of Agusan del Sur. The province of Agusan del Sur can be reached both by land and water transportation. The Butuan-Davao National Highway (Daang Maharlika Highway) or the north-south backbone route served as the inter-regional road linking Davao Region and Caraga Region. It is the busiest road serving the region.

Agusan derived its name from the local dialect, agasan, meaning "where the water flows", reffering to the Agusan River that splits the land and meanders south to north in a 250-kilometre (160 mi) rush to Butuan Bay. It is third largest river in the country and served as highway for the Spanish colonizers in gaining access to inner noertheastern Mindanao.