The law, AB144, makes it a misdemeanor to carry an exposed and unloaded gun in a public place.
Top state law enforcement officials supported the law. Opponents said the bill violates the public's Second Amendment rights.
Brown had more than 100 bills to sign or veto before a midnight deadline.
The governor signed a bill that makes it illegal for teenagers under 18 to use tanning beds. Right now, it's illegal for those 14 and under to use them, but teens 15 to 17 can do so with their parents' permission. The bill makes California the first state to set a higher age limit.
Brown also signed a new measure requiring health insurance companies in California to provide coverage for autism.
He also signed an STD vaccinations bill that allows children as young as 12 to get the HPV vaccination without their parents' consent.
Brown vetoed a bill requiring extra warning for women whose breast tissue is dense enough to mimic cancers in mammograms.
He also vetoed a bill allowing public universities to use race in their undergraduate and graduate admissions.
The governor said no to a bill requiring law enforcement officers to get a warrant before they search through electronic devices of people they arrest.