He made the announcement late Tuesday morning on his website. Brown is the only Democratic candidate so far.
He was the state's youngest governor when he was first elected in 1976. Brown would become the state's oldest governor if elected in November.
Brown's announcement will come one day after insurance commissioner Steve Poizner made his run for governor official. The Republican filed paperwork with the Santa Clara registrar of voters on Monday. Tuesday, he launched two television ads attacking his Republican opponent Meg Whitman.
Whitman is leading Poizner in the polls for the Republican nominee. The former C.E.O. of eBay has spent $39-million dollars of her own money to campaign for Governor.
