was a district located in Ōita Prefecture, Japan.
As of 2003, the district had an estimated population of 12,739 and the density of 48.85 persons per km<sup>2</sup>. The total area was 260.79 km<sup>2</sup>.
Until March 30, 2005, the district had two towns:
- Ajimu
- Innai
Merger
On March 31, 2005 - the towns of Ajimu and Innai were merged into the expanded city of Usa. Therefore, Usa District was dissolved as a result of this merger.