The 48-year-old made it official on Tuesday at City Hall in Philadelphia. He's also scheduled to make announcements later in the day in Harrisburg and Pittsburgh.
Onorato pledged to improve the state's business climate, increase educational opportunities and preserve green space.
Onorato will compete with Philadelphia businessman Tom Knox, Scranton Mayor Chris Doherty and Montgomery County Commissioner Joe Hoeffel for the 2010 nomination. Onorato was formerly a Pittsburgh councilman and county controller.
Attorney General Tom Corbett will face U.S. Rep. Jim Gerlach of Chester County for the Republican nomination.