Boxer, 74, said neither age nor partisanship in Congress were factors in her decision.
Boxer, the junior senator from California, was first elected to the Senate in 1992. She served in the House for a decade before that. She was a strong supporter of environmental protections, abortion rights and gun control.
Boxer, a liberal, was a favorite to retain her seat had she run. She said she would work to ensure her Senate seat would remain with Democrats. The other seat from California is occupied by Sen. Dianne Feinstein, also a Democrat.