More than seven million shares exchanged hands in single trade in the afternoon, representing about 18 per cent of the company's outstanding class B shares.
The transaction was unusual for the company's stock, which generally trades in low numbers.
Postmedia class B shares ended the session down 8.6 per cent, or 15 cents, at $1.60 on total volume of 7.1 million shares.
A spokeswoman for the company, which owns several newspapers including the National Post, said she was unaware of any reason for the significant exchange in stock.
The company has been struggling to turn around its money-losing operations with staff reductions and office closures.
In its third-quarter, the Postmedia's losses deepened to $112.2 million on a $93.9-million impairment charge from a production facility that is up for sale.
Earlier this month, the company announced plans to sell two properties in British Columbia and Alberta in a bid to reduce costs and pay down debt.
In July, Moody's Investors Service downgraded Postmedia's rating to negative from stable due to the media company's declining cash flow as it transitions to a digital business.
Postmedia also owns the Vancouver Sun and the Ottawa Citizen.
The company is scheduled to report its latest financial results on Oct. 23.