California's 21st State Assembly district is one of 80 California State Assembly districts.

It is currently represented by Democrat Diane Papan of San Mateo.

The district includes most of San Mateo County, California.

Most of San Mateo County – 64.50% Due to redistricting, the 21st district has been moved around different parts of the state.

The current iteration resulted from the 2011 redistricting by the California Citizens Redistricting Commission.