Assemblyman Tim Donnelly (R-Twin Peaks) was stopped at a TSA screening point, before boarding a flight to Sacramento.
The TSA called airport police, who found a Colt .45 with four rounds in the bag, along with another magazine holding five rounds.
Donnelly, who is an outspoken gun rights advocate, says he simply forgot the gun was there.
"You hear stories about this and you think, 'What is this person thinking? What kind of an idiot would carry a gun into an airport?' And it was just really a regrettable mistake," said Donnelly.
He said he had it close by because of death threats he has received from a referendum drive he is spearheading to overturn a recent state law that allows illegal immigrant college students to apply for public financial aid. He told reporters at the state Capitol he placed it in the briefcase because he didn't want his wife to see that he had the firearm out.
"So I do tend to always be armed. The issue is strictly one that I completely forgot, coming back to work this morning, that it was in that briefcase," he said.
Donnelly did not respond when reporters asked if he had a concealed weapons permit.
Donnelly was cited and the gun was confiscated.
The Associated Press contributed to this report.
