The Roman Catholic Diocese of Buxar (Latin: Dioecesis Buxarensis) in India was created on December 12, 2005.

The parish church "Mary Mother of Perpetual Help" in Buxar is the cathedral for the diocese.

The diocese covers an area of 11,311 km² of the Patna state, comprising the districts Buxar, Bhojpur, Bhabua, and Rohtas.

The Sone River marks the boundary between the dioceses Patna and Buxar to the southeast.

Other dioceses bordering are Varanasi to the west, Muzaffarpur to the north, and Daltonganj to the south.