The Christian Social Union in Bavaria (German: , CSU) is a Christian democratic and conservative political party in Germany. Having a regionalist identity, the CSU operates only in Bavaria while its larger counterpart, the Christian Democratic Union (CDU), operates in the other fifteen states of Germany. It differs from the CDU by being somewhat more conservative in social matters. Founded in 1945, the CSU is considered the de facto successor of the Weimar-era Catholic Bavarian People's Party.
At the federal level, the CSU forms a common faction in the Bundestag with the CDU which is frequently referred to as the Union Faction (die Unionsfraktion) or simply CDU/CSU. The CSU has had 44 seats in the Bundestag since the 2025 federal election, making it currently the sixth largest of the seven parties represented and the largest parliamentary faction as part of CDU/CSU with 208 seats. The CSU is a member of the European People's Party and the International Democracy Union.