Pupatello made the announcement Thursday morning in Toronto when she formally declared she would seek to succeed Premier Dalton McGuinty, who is relinquishing his role as Liberal leader.
Pupatello's number one priority is job creation, she said at a news conference at Ryerson University.
"The reason I'm doing this is I want the opportunity to have not just the Ontario government [but] all parties, all of us talking about jobs," she said.
"It's what I haven't heard much of this year and what I think we need to revert back to."
McGuinty prorogued the legislature when he announced his decision to quit, drawing criticism that the move was anti-democratic, had stalled the legislative agenda, and was a move aimed at avoiding controversies swirling around the government.
Finance Minister Dwight Duncan has offered to give up his seat in Windsor-Tecumseh and let Pupatello run in a byelection if she becomes the Liberal leader, she said. Duncan has already endorsed Pupatello for Liberal party leader, as has transportation minister Bob Chiarelli.
Pupatello decided not to run for re-election last year, saying it was time for a change. Liberal Teresa Piruzza now holds her former seat of Windsor West.
If she wins the race, Pupatello would make history as Ontario's first female premier. Her husband, lawyer Jim Bennett, is the former leader of the Newfoundland Liberal Party.
Pupatello is the third contender to formally enter the leadership race, following Toronto Centre MPP Glen Murray and Don Valley West MPP Kathleen Wynne, who both launched leadership bids last weekend.
The Liberal leadership convention is to be held in Toronto on the weekend of Jan. 25. It is not immediately clear how long it would take to hold a byelection after that if Pupatello is elected leader and becomes premier.
Pupatello tagged as favourite
Later Thursday, Pupatello will go to a rally and membership drive at Fogolar Furlan Club in Windsor.
University of Windsor political science professor Lydia Miljan said Pupatello has to be the leading candidate to take the leadership at this point.
"She's sort of the big-name brand. So I think in terms of a race, if it was just those three people, or even some others, she's probably the one most likely to prevail," Miljan said "So I can see her being the leader of the Liberals. The question then is: Can she form a government?"
Howard Pawley, another University of Windsor political science professor, said Pupatello has distanced herself from the recent gas plant scandal to hit the Liberals.
"What I think may have caused some others to have just dropped out, she has not been associated with that, so I think that is a positive thing for her," Pawley said.
Tecumseh Mayor Gary McNamara said he will support Pupatello in her bid.
He called Pupatello "well respected" and "a dynamo."
"The princess warrior as they call her," McNamara said. "I think she's the right leader to lead the party."