The Bishopric of Culm (German: Bistum Culm; Polish: Diecezja chełmińska) was a Roman Catholic diocese in Chełmno Land (Culm land), founded in medieval Prussia in 1243 and disbanded in 1992.
It was founded in 1243 by the papal legate William of Modena in the Monastic state of the Teutonic Knights, along with the three other bishoprics, Bishopric of Warmia, Samland (in Sambia) and Pomesania. Initially Culm was a suffragan to the Archdiocese of Riga and had its seat in Chełmża (Culmsee). From 1257 to 1773 its seat was the castle in Lubawa. Since 1466 Culm diocese was part of Polish Royal Prussia. After 1525 Culm incorporated southern parts of the Pomeasanian diocesan area (with Łasin and Nowe Miasto), which happened to be in the Chełmno Voivodeship. Whereas western Pomesanian diocesan area in the Malbork Voivodeship was administered by Culm, but officially maintaining its naming.
After Riga's dissolution in 1566 the bishops of Culm attended the councils of the Ecclesiastical Province of Gniezno. This practice was recognised by the Holy See by the Bull De salute animarum in 1821, when Culm became de jure a suffragan of the Archdiocese of Gniezno. Culm diocese was enlarged on that occasion (Górzno, Krajna and Soldau). Since 1772 Culm diocese was part of the Kingdom of Prussia and became a part of Germany in 1871. In 1824 the see had moved to Pelplin. In 1920 Culm became part of the Second Polish Republic. On 1 May 1923 the Holy See disentangled the deaneries in Bütow (Bytow) and Lauenburg in Pomerania (Lębork) from Culm and transferred them to the new Apostolic Administration of Tütz. As part of the reorganisation of the Catholic Church in the Third Polish Republic Culm was disbanded in 1992 by Pope John Paul II and replaced by the Diocese of Pelplin.