A statement from her campaign Thursday announced the Orange County Democrat picked up endorsements from Reps. Mark Takano of Riverside, Grace Napolitano of El Monte, Raul Ruiz of Palm Desert and Scott Peters of San Diego.
The 10-term congresswoman has endorsements from about a dozen Democratic House members from California.
Boxer announced in January she wouldn't seek a fifth term, setting the stage for the 2016 contest.
Attorney General Kamala Harris, a Democrat, entered the race in January.
Three Republicans are seeking the seat, Assemblyman Rocky Chavez of Oceanside and former state GOP leaders Duf Sundheim and Tom Del Beccaro.
Democrats are favored to hold the seat.