The Roman Catholic Diocese of Thunder Bay (Latin: Dioecesis Sinus Tonitralis) is a Latin suffragan in the ecclesiastical province of the Metropolitan Archdiocese of Toronto, in Ontario, Canada.

Its cathedral episcopal see is St. Patrick's Cathedral, dedicated to St. Patrick, in Thunder Bay, Ontario.

As per 2014 it pastorally served 81,400 Catholics (31.0% of 262,600 total) on 220,000 km2 in 43 parishes with 46 priests (37 diocesan, 9 religious), 36 deacons and 16 lay religious (9 brothers, 7 sisters) .

(all Roman Rite)