CBC News has learned Pupatello will announce her leadership on Thursday bid for the Ontario Liberal Party.
She will meet with media in Toronto on Thursday morning and then head to Windsor's Fogolar Furlan Club, where she was previously general manager. There, she'll rally with supporters.
Pupatello would be 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.
Outgoing Finance Minister Dwight Duncan may have prematurely let the cat out of the bag late Tuesday night about Pupatello's pending leadership bid.
“It will be fun watching Hudak concede to Sandra and Harper concede to Justin at the next general elections,” Duncan tweeted.
Duncan then issued a statement Wednesday afternoon.
"I will support my friend and colleague, Sandra Pupatello, if she decides to run for leader of the Ontario Liberal Party," Duncan wrote. "I believe she would serve the party well, bringing years of passion and experience to the job.
"If chosen as leader, I also believe she has what it takes to win the next general election and to lead this province as premier."
A second source then confirmed to CBC News that Pupatello will make her announcement Thursday.
Pupatello represented the Windsor West riding from 1995 to 2011, holding several cabinet positions.
She left provincial politics in 2011 and took at job at PricewaterhouseCoopers.
Also Wednesday, Mississauga South MPP Charles Sousa announced he will make "an important announcement about the future of Ontario" on Saturday.
Health minister won't enter race
Health Minister Deb Matthews announced Wednesday that she will not run for the leadership.
A handful of other cabinet ministers have also taken a pass on a leadership run, including Finance Minister Dwight Duncan, Education Minister Laurel Broten, Energy Minister Chris Bentley and Brad Duguid, the minister of economic development and innovation.
Ottawa Centre MPP Yasir Naqvi, the party president and George Smitherman, the former MPP and cabinet minister, have also said they will not pursue the leadership.
Candidates have until Nov. 23 to jump into the race. Other Liberals still considering a run include Harinder Takhar, Eric Hoskins and former MPP Gerard Kennedy.
The Liberals will choose their leader at a convention in Toronto on the weekend of Jan. 25.
The leadership convention was called after McGuinty announced last month that he was stepping down as Liberal leader.
He also prorogued the legislature and has left it to his successor to decide when it will be recalled.