The California Democrat said Tuesday states that don't screen people who want to carry concealed weapons aren't protecting their citizens.
Boxer's remarks were prompted by last week's shooting in Tucson, Ariz., in which six people were killed and Rep. Gabrielle Giffords and 13 others were wounded.
Arizona law allows any U.S. citizen 21 or older to carry a concealed weapon without a permit.
Boxer also says she supports a proposal by Sen. Frank Lautenberg, D-N.J., to ban high-capacity ammunition clips.
She spoke at a news conference in the Riverside County district attorney's office.