Capriles is now running as the incumbent in Miranda state against Chavez's former vice president, Elias Jaua. The elections on Dec. 16 will decide gubernatorial posts nationwide.
Jaua vowed to win in a message on Twitter, saying: "We're going to get Miranda back!"
Capriles signed up to run Thursday and told a crowd in the town of Los Teques that his government showed "efficiency and transparency" and that his opponent would bring corruption.
The opposition leader told his supporters that no one should feel pessimistic after the loss in the presidential vote. Chavez won by 11 percentage points, his narrowest re-election margin to date.