The Liberal Quebec senator cited personal and family reasons for his withdrawal from political life three years before the end of his term.
A family member is seriously ill.
His resignation will take effect on Friday, his 72nd birthday.
The Trudeau-era cabinet minister served 12 years in Parliament and six as a senator.
He was communications minister when he introduced the landmark Access to Information Act on July 17, 1980, although it only passed in 1982.
He had also been solicitor general and was a rising star in the Trudeau government; but he was forced to temporarily resign on Jan. 27, 1978, after acknowledging in public that he had forged the signature of his girlfriend's husband on a hospital form so she could get an abortion.