Knox ran second to Nutter among seven candidates in the 2007 Democratic mayoral primary, after spending close to $10.6 million of his personal fortune on the effort.
He said he was prepared to spend the same amount again, "but I hope I don't need to. I'm a lot better known now that I was. . . . The answer is, I'll be competitive."
Knox, who made his fortune in the insurance business, was leading in the pre-primary election polls in 2007 until several of his rivals focused their criticism on his inexperience as a government official.
Knox worked 19 months for former Mayor Ed Rendell as deputy mayor for management and productivity. He said he led efforts to reduce the city's health care expenses, and consolidate its automobile fleet and information and technology systems.
In the last two weeks of the primary campaign, Nutter surged ahead, finishing with 106,805 votes, 36.6 percent, vs. 71,731 for Knox, 24.6 percent.
Knox, 72, lives in Center City with his wife, Linda.
His announcement Thursday was distributed by Frank Keel, a longtime spokesman for John Dougherty, the powerful electricians' union head and Democratic ward leader who was Knox's biggest political backer in 2007.
Other Democrats mentioned prominently as potential mayoral candidates in 2015 include State Sen. Anthony Williams, City Council members Bill Green, Jim Kenney and Darrell Clarke, and city controller Alan Butkovitz.
Contact Bob Warner at 215-854-5885 or firstname.lastname@example.org.