Campoli was an unrestricted free agent after the Chicago Blackhawks walked away from his $2.5 million arbitration award earlier this summer.
He had 21 points (4 goals, 17 assists) in 77 games last season with Ottawa and Chicago.
"Mr. Campoli was in a difficult situation because by the time Chicago walks away after arbitration you're almost into August, and then the market is a lot different," Montreal general manager Pierre Gauthier said. "There are not too many buyers, and if there are buyers you don't know what kind of resources they have, so these things evolve but we're happy they evolved with us today."
The Canadiens are awaiting the return of All-Star defenseman Andrei Markov from a second knee operation in two years. Markov had a setback in his rehabilitation and isn't expected to be ready for the beginning of the regular season.
"He's progressing very well, the knee's in very good health and he keeps moving along his rehabilitation," Gauthier said. "And so we'll see if he'll be quite ready or not, but he shouldn't miss too many games."
Campoli, who began his career with the New York Islanders, has 135 points in 397 NHL games.
Yannick Weber and former Dallas defenseman Jeff Woywitka are already in the mix along with European rookies Rafael Diaz and Alexei Yemelin for limited spots on the Montreal blue line, which already had five experienced defencemen in place.
"All we're doing is adding a solid player to the group," Gauthier said. "As we know from the past years, particularly last year, to carry seven or eight defensemen on your NHL team at any time is the right thing to do."
The Canadiens made 10 cuts Monday to reduce their training camp roster to 45 players. Prospects Olivier Archambault, Nathan Beaulieu and Morgan Ellis were returned to their junior teams. Forwards Alexander Avtsin, Andrew Conboy and Dany Masse, defensemen Mark Mitera and Joe Stejskal and goalies Robert Mayer and Peter Dekmas were assigned to Hamilton of the AHL.
Pierre LeBrun is an NHL writer for ESPN.com. Information from The Associated Press was used in this report.