The Roman Catholic Archdiocese of Onitsha (Latin: Onitshan(us)) is the Metropolitan See for the ecclesiastical province of Onitsha in Anambra State, Nigeria.

On 25 July 1889, the Apostolic Prefecture of Lower Niger was established out of an area that previously formed part of the larger Apostolic Vicariate of Benin Coast.

On 16 April 1920, the Prefecture was promoted as the Apostolic Vicariate of Southern Nigeria.

On 9 July 1934, the Vicariate was renamed from Southern Nigeria to Onitsha-Owerri.

The seat of the archbishop is the Holy Trinity Basilica in Onitsha.