The Ajay River originates on a small hill about 300 metres (980 ft) high, southwest 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. Total length of the Ajay is 288 kilometres (179 mi), out of which 152 kilometres (94 mi) is in West Bengal.
The catchment area of Ajay River is 6,000 square kilometres (2,300 sq mi).
The important tributaries of the Ajay are Pathro 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 palas trees till recent times when mining and other activities led to the clearing of forests.
In the history of ancient India edited by Mc Krindle, there was a river called Amystis that flows past a town called Katadupa, in the days of Megasthenes. Another historian Wilfred opines that Amystis is modern-day Ajay. In recent times there has been exploration of the remains of an ancient civilisation similar to that of the Indus Valley Civilisation at Pandu Rajar Dhibi in the lower Ajay valley.
There have been at least 14 recorded floods in the 20th century in the Ajay river. The lower reaches of the river have embankments to prevent flooding.