It will prevent people under 21 from buying rifles and other types of guns. State law already bans people under 21 from buying handguns.
The new law exempts law enforcement, members of the military and people with hunting licenses from the restriction.
RELATED: California legislature passes bill that will raise gun buying age from 18 to 21
It was one of dozens of bills Brown took action on.
Democratic Sen. Anthony Portantino pointed to the shooting at a Florida high school earlier this year that killed 17 people as the reason for his bill banning gun sales and transfers to people under 21.
"I was determined to help California respond appropriately to the tragic events our country has recently faced on high school campuses," Portantino said in a statement. "I feel it is imperative that California leads when Washington refuses to act."
Brown also signed a bill to prohibit gun ownership for people who have been hospitalized or otherwise placed on an involuntary psychiatric hold for risk of hurting themselves or others twice in one year. That law would let those people ask a court every five years to return their guns.
He also signed a bill to ban people with certain domestic violence misdemeanors from owning guns for life.
In addition to the gun-related bills, he vetoed a measure that would have let bars in some cities serve alcohol until 4 a.m., which he said would result in more drunken driving.