The Lakhandei River is a river in southern Nepal and in the state of Bihar in India. It is a main tributary of the Bagmati River.
In Nepal, the river basin lies in the Sarlahi District, and the river originates in the Sivalik Hills. The river enters India in the Sitamarhi district of Bihar, and flows through the town of Sitamarhi. It then enters the Muzaffarpur district, and joins the Bagmati at Katra (within the Muzaffarpur district).
The Lankhandei is known for flooding.