MacInnis starred for the Calgary Flames and Blues during his stellar 23-year NHL career, retiring after the 2003-04 season. He played just three games in his final campaign because of an eye injury.
"His knowledge and awareness of the game are well respected in the NHL," Blues president John Davidson said in a release. "He is a first class person and will play a major role in our quest to build a winner in St. Louis."
MacInnis ranks third all-time among NHL defencemen in goals (340), assists (934) and points (1,274), and ranks sixth with 1,416 games played. He also ranks 12th all-time among NHL players in assists and is tied for 29th in points.
"The club is important to me," said MacInnis, "and it will be our priority to put a competitive team on the ice working towards the Stanley Cup."
MacInnis, from Port Hood, N.S., was drafted by Calgary 15th overall in 1981 and made his Flames debut later that year. He helped Calgary to its only Stanley Cup championship in 1989 and spent 13 seasons there.
The Blues acquired MacInnis after the 1993-94 season and he spent 10 seasons in St. Louis, where he won the Norris Trophy in '99.