The contract is a one-year deal worth $1.875 million.
Dunn, 24, finished the 2019-20 season with 23 points (nine goals, 14 assists) in 71 games. He was the only Blues defenseman to play in all 71 games.
Dunn has played in 224 NHL games while recording 82 points (26 goals, 56 assists). Dunn was originally drafted by the Blues in the second round of the 2015 NHL Draft.