Rieder joined the Flames on a professional tryout ahead of the preseason.
The 26-year-old failed to score a goal in 67 games with the Edmonton Oilers in 2018-19 after signing with them as a free agent last July. Oilers CEO Bob Nicholson openly criticized the German winger after the campaign, telling season-ticket holders that if Reider had "scored 10 or 12 goals, we'd probably be in the playoffs."
A fourth-round pick of the Oilers in 2011, Rieder spent the first three seasons of his NHL career with the Arizona Coyotes from 2014-17. He played another 58 contests with them in 2017-18 before being shipped to the Los Angeles Kings at the trade deadline. He'd never scored fewer than 12 goals in a campaign prior to last season.