The Ajay River originates on a small hill about 300 metres (980 ft) high, south-west of Munger in Bihar. It then flows through Jharkhand and enters West Bengal at Simjuri, near Chittaranjan. It first forms the border between Bardhaman District and Jharkhand and then between Bardhaman District and Birbhum District, and finally, it enters Katwa subdivision of Bardhaman district at Nareng village in Ketugram police station. It then joins the Bhagirathi River at Katwa Town. The total length of the Ajay is 288 kilometres , out of which 152 kilometres are in West Bengal.
The catchment area of Ajay River is 6,000 square kilometres
The important tributaries of the Ajay are Panthro and Jayanti in Jharkhand and Tumuni and Kunur in Bardhaman district of West Bengal.
The upper reaches of the Ajay pass through hilly regions with laterite soil. It is only from Ausgram in Bardhaman district that the Ajay flows through alluvial plains. The Ajay valley was densely forested with sal, Piyasal and palms trees till recent times when mining and other activities led to the clearing of forests.