The Christian Social Union in Bavaria (German: Christlich-Soziale Union in Bayern^ⓘ, 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, following Catholic social teaching. 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.

Party leader Markus Söder serves as Minister-President of Bavaria, a position that CSU representatives have held from 1946 to 1954 and again since 1957. From 1962 to 2008 and from 2013 to 2018, the CSU had the absolute majority in the Bavarian Landtag.

The CSU has led the Bavarian state government since it came into existence in 1946, save from 1954 to 1957 when the SPD formed a state government in coalition with the Bavaria Party and the state branches of the GB/BHE and FDP.

Initially, the separatist Bavaria Party (BP) successfully competed for the same electorate as the CSU, as both parties saw and presented themselves as successors to the BVP. The CSU was ultimately able to win this power struggle for itself. Among other things, the BP was involved in the "casino affair" under dubious circumstances by the CSU at the end of the 1950s and lost considerable prestige and votes. In the 1966 state election, the BP finally left the state parliament.

Franz Josef Strauß (1915–1988) had left behind the strongest legacy as a leader of the party, having led the party from 1961 until his death in 1988. His political career in the federal cabinet was unique in that he had served in four ministerial posts in the years between 1953 and 1969. From 1978 until his death in 1988, Strauß served as the Minister-President of Bavaria. Strauß was the first leader of the CSU to be a candidate for the German chancellery in 1980. In the 1980 federal election, Strauß ran against the incumbent Helmut Schmidt of the Social Democratic Party of Germany (SPD) but lost thereafter as the SPD and the Free Democratic Party (FDP) managed to secure an absolute majority together, forming a social-liberal coalition.

Before the 2008 elections in Bavaria, the CSU perennially achieved absolute majorities at the state level by itself. This level of dominance is unique among Germany's 16 states. Edmund Stoiber took over the CSU leadership in 1999. He ran for Chancellor of Germany in 2002, but his preferred CDU/CSU–FDP coalition lost against the SPD candidate Gerhard Schröder's SPD–Green alliance.

In the 2003 Bavarian state election, the CSU won 60.7% of the vote and 124 of 180 seats in the state parliament. This was the first time any party had won a two-thirds majority in a German state parliament. The Economist later suggested that this exceptional result was due to a backlash against Schröder's government in Berlin. The CSU's popularity declined in subsequent years. Stoiber stepped down from the posts of Minister-President and CSU chairman in September 2007. A year later, the CSU lost its majority in the 2008 Bavarian state election, with its vote share dropping from 60.7% to 43.4%. The CSU remained in power by forming a coalition with the FDP. In the 2009 general election, the CSU received only 42.5% of the vote in Bavaria in the 2009 election, which by then constituted its weakest showing in the party's history.