Tavares and Marleau replace forwards Leo Komarov and Tyler Bozak, who signed elsewhere as free agents. Rielly, a 24-year-old defenseman, was an alternate captain last season.
"I think first for management, the organization, to feel that as a new guy coming in that I can make that kind of impact right away on the group and be looked upon as a leader is something I'm very thankful for and don't take for granted," Tavares said. " I understand what that means and what that represents and how important that is, just not take it for granted. Go out there, be myself and do the best I can for the hockey club."
Toronto general manager Kyle Dubas had told TSN he didn't think the Maple Leafs would have a captain this season.
"Not from the outset, I think," Dubas said. "It's a very important honor to be the captain of the Toronto Maple Leafs or any team that has a strong history."
Center Auston Matthews was thought to be in line for the captaincy but he will not wear a letter this season.
Tavares signed a seven-year, $77 million contract (average annual value $11 million) with the Maple Leafs on July 1. The 27-year-old center was captain of the New York Islanders the previous five seasons.
Marleau signed with the Maple Leafs on July 2, 2016. The 38-year-old forward was captain of the San Jose Sharks from 2003-09.
The Maple Leafs have not had a captain since defenseman Dion Phaneuf was traded to the Ottawa Senators on Feb. 9, 2016.
Bozak signed with the St. Louis Blues, and Komarov signed with the Islanders.
