The Diocese of Thanh Hóa () is a Roman Catholic diocese of Vietnam. The bishop since 2018 is Joseph Nguyá» n ÃÂức Cðá»Âng. Of the three million persons living in its area about 3% are Roman Catholics.
The diocese covers an area of 11,168 km<sup>2</sup>, and is a suffragan diocese of the Archdiocese of HàNôi. It was erected on May 7, 1932 as a Vicariate Apostolic. On November 24, 1960 it was elevated to a diocese.
Saint Anne Cathedral in Thanh Hoa city has been assigned as the Cathedral of the diocese.