The newspaper is seeking an appeal of an Ontario Court of Appeal decision last month that ruled its staff must turn over a document from a confidential source in 2001 to the Royal Canadian Mounted Police.
The document pertained to the newspaper's probe of alleged conflicts of interest in former Prime Minister Jean Chretien's government.
The Ontario court ruled it's up to the media to show there is a greater benefit to keeping information confidential than to disclose it when it's wanted by police.
Doug Kelly, editor-in-chief of the National Post, said the request for a Supreme Court ruling was based on "functioning democracy" and the media's role in it.
"Stripping away a reporter's ability to protect a credible source will spread a chill across journalism in this country and severely hamper our ability to expose wrongdoing and bring critical facts to light," he said.