In 1466, the region was confirmed as part of Poland, and the diocese was agreed to pass to the Archdiocese of Gniezno.[3]
After 1525 Chełmno incorporated southern parts of the Pomesanian 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 Chełmno, but officially maintaining its naming.
After Riga's dissolution in 1566 the bishops of Chełmno attended the councils of the ecclesiastical province of the metropolitan of Gniezno. This practice was recognised by the Holy See by the Bull De salute animarum in 1821, when Chełmno became de jure a suffragan of the Archdiocese of Gniezno. Chełmno diocese was enlarged on that occasion (Górzno, Krajna and Działdowo).
Pope Pius XI decided to separate 18 parishes in the territory the Free City of Danzig west of the Vistula from the diocese and to establish an Apostolic Administrator of the Free City of Danzig on 24 April 1922, which was directly subordinated to the Pope.[5]
In 1925 a concordat between Poland and the Holy See was signed and the Apostolic Administrator was now supposed to be subordinated to the Nuncio of Warsaw, which caused protests among the local populace. Thus, the Pope established the sui iurisDiocese of Danzig on 30 December 1925 and appointed Edward O'Rourke as the first Bishop on 2 January 1926.[5] The deanery of Pomesania in that eastern part of West Prussia which remained with Germany after the 1920 East and West Prussian plebiscites was transferred to the Diocese of Warmia in 1925.
As part of the reorganisation of the Catholic Church in the People’s Republic of Poland in 1972, also accounting for changes of political border in 1945, the diocese of Gdansk was enlarged on the expenses of Chelmno diocese, whereas the latter gained parishes previously part of the Berlin diocese and the Prelature of Schneidemühl (Piła)
Górski, Karol (1949). Związek Pruski i poddanie się Prus Polsce: zbiór tekstów źródłowych (in Polish and Latin). Poznań: Instytut Zachodni. pp.88, 99, 206, 217.