The wealthy businessman tweeted, "I'm running," and added a link to a video on his website, brucerauner.com.
On Wednesday, the venture capitalist went without being recognized on busy State Street in Chicago's Loop. He's never run for or held public office, which Rauner, a Republican, thinks will help convince voters to support him.
"An independent person who's financially not dependent on any outside groups who can bring a fresh problem-solving perspective can make a huge difference in Springfield," Rauner said.
To announce his candidacy, Rauner used a slick YouTube video outlining the state's fiscal problems. Later, he did one-on-one interviews at media outlets but not the traditional announcement day press conference.
"We want to maximize the breadth and the depth of our announcement impact to raise my profile as much as possible," Rauner said.
Over the weekend, Illinois State Treasurer Dan Rutherford, also a Republican, announced his candidacy.
"I'm the kind of guy who walks parades, shakes hands and looks people in the eye," Rutherford said. "I do understand he's a billionaire from Chicago. And perhaps his approach in how you communicate with individuals in a campaign circuit is different than mine."
"It doesn't, why does that matter? I'm very successful in business. I know how to build companies," Rauner said when asked if he were a billionaire.
Other Republicans expected to formally announce candidacies include state senators Bill Brady and Kirk Dillard, who finished first and second in the 2010 primary.
Rauner says his 2014 campaign will target other Republican politicians, as well as Democrats whom he says have made too many unaffordable deals with public sector unions.
"They own most of the Democratic Party through their political donations and their campaign patronage workers and unfortunately for us they control a lot of the Republicans in Springfield, too," Rauner said.
A source close to Rauner estimates the candidate could spend anywhere from $10 to $20 million during the primary campaign. Rauner said his ads could begin airing on television this month.