The governor announced signing other gun-related legislation on Friday but rejected the centerpiece bill, which would have imposed the nation's toughest restrictions on gun ownership.
Brown's veto message for SB374 said it was too far-reaching. The bill by Democratic Senate President Pro Tem Darrell Steinberg would have outlawed future sales of an entire class of weapons that includes most assault rifles.
It was lawmakers' latest attempt to close loopholes that have let manufacturers work around previous assault weapon bans.
Gun-rights groups had threatened to sue if the semi-automatic weapons ban became law.