Correctional Service Canada says Earl William Davenport was found unresponsive in his cell on Monday and could not be revived.
A Correctional Service spokesman says foul play is not suspected.
Davenport, who was 56, had been serving an indeterminate sentence since November 1987.
He was convicted of first- and second-degree murder in the deaths of his sister-in-law and her friend.
Laura Davenport and Susan Hornbeck were stabbed to death in an attack in Hamilton in 1986.
As in all cases involving an inmate's death, the police and the coroner have been notified, and Correctional Service Canada will review the circumstances. (CHQR, The Canadian Press)
