California's 1st state senatorial district is one of 40 California State Senate districts. The district is represented by of .
The district includes most of the California Cascades, northern Sierra Nevada and the Sacramento Valley, comprising Siskiyou, Modoc, Lassen, Shasta, Tehama, Plumas, Glenn, Colusa, Butte, Sutter, Yuba, Nevada, and Sierra Counties, along with eastern Placer County.
Due to redistricting, the 1st district has been moved around different parts of the state. The current iteration resulted from the 2021 redistricting by the California Citizens Redistricting Commission.