The Diocese of Saint Joseph at Irkutsk (Latin: Ircutscana Sancti Iosephi) is a Latin Church diocese of the Catholic Church located in the city of Irkutsk, which is part of the ecclesiastical province of the Mother of God at Moscow in Russia.

The Diocese of Irkutsk is the largest geographical Catholic bishopric on earth, covering an area of 9,960,000 km2.

The Catholic population of the diocese is estimated to be about 53,000.

The bishop is Cyryl Klimowicz, a Pole.

The diocese is divided into five deaneries: Irkutsk, Krasnoyarsk, Yakutsk, Vladivostok and Magadan.