The 1938 United States Senate election in California was held on November 2, 1938. Incumbent Democratic Senator William Gibbs McAdoo ran for a second term, but was defeated by Sheridan Downey in the Democratic primary. Downey went on to defeat Philip Bancroft in the general election.
Senator McAdoo had the support of President Franklin Roosevelt and George Creel.
Downey was supported by his 1934 running mate Upton Sinclair and pensioner advocate Francis Townsend.
Although he did not cross-file to appear on the ballot, Sheridan Downey did receive write-in votes.