The 28-year-old joins the Blues after one season with the Colorado Avalanche.
Saad recorded 15 goals and 24 points in 44 regular-season games and added an another seven goals in 10 playoff games in 2020-21.
After losing Jaden Schwartz to Seattle and Mike Hoffman to Montreal on the opening day of free agency, plus Vladimir Tarasenko’s unclear future with the team, the Blues needed to add a top-six calibre winger and Saad fit the bill.
The Avalanche recently gave captain Gabriel Landeskog and star blueliner Cale Makar lucrative long-term deals and general manager Joe Sakic said of Saad when speaking with reporters on Wednesday: “Unfortunately with the way the salary cap is, we weren't able to keep him.”
Saad has scored more than 20 goals in five of the past seasons. He has 184 goals and 371 points in 632 career regular-season games split between the Blackhawks, Blue Jackets and Avalanche.