The debate is set for Sept. 27 and will be the only head-to-head meeting of the party leaders ahead of the Oct. 6 provincial election.
Green Leader Mike Schreiner says the media consortium in charge of the televised debate notified his party of the date, but the Greens were not invited.
The CBC's Jeff Keay, speaking for the consortium, said the group's decisions are made on a case-by-case basis, but several factors were considered, including the lack of a Green seat in Ontario's legislature.
"The consensus decision of the broadcasting consortium was to go with the three leaders of the parties that are represented in the legislature," Keay said Friday.
Schreiner, however, says his party has earned a place at the table.
"We are a strong, united, organized province-wide political party that has a great deal of support," he said.
Green officials note they are running a full slate of 107 candidates across Ontario and won 354,897 votes in the 2007 provincial election — six times that of all the minor parties combined.
The party has launched a petition website LetMikeSpeak.ca and so far has gathered more than 6,000 signatures, Schreiner said. There are also non-partisan campaigns on Facebook calling for the Greens inclusion.
There's also precedent, Schreiner added, pointing out that Greens have taken part in leaders' debates in Prince Edward Island, New Brunswick and British Columbia without holding a seat.
Schreiner was joined Friday by national Green party Leader Elizabeth May for a news conference at the Ontario legislature. The pair argued the Green perspective needs to be heard at the debate.
May was excluded from the federal leaders' debate earlier this year since her party did not have a seat in the House of Commons.
She won her B.C. riding anyway, becoming the first Green party candidate to be elected federally.
"In a democracy there are no seats in the house at the time of the debates," May told the news conference Friday. "Everybody is running for a position."
While May was unable to debate the other federal leaders this spring, she did appear at the 2008 debate, Schreiner pointed out in an interview Friday.
That appearance helped raise her profile and "probably had something to do with the fact that she was elected in 2011," he said.
Meanwhile, the Greens have sent letters to the other leaders, asking for their support, Schreiner said, adding NDP Leader Andrea Horwath has offered hers.
On Friday, Premier Dalton McGuinty said he would leave decisions about the debate to the consortium.
"There's a group of folks ... a process associated with making those kinds of decisions and I have my faith in that process," he said.