The 32-year-old has played 278 matches for the Blades since arriving from Coventry City on a free transfer in 2016, registering 16 goals and 48 assists in the process, winning their Player of the Season prize in his first two campaigns.
Since arriving at Bramall Lane, Fleck played a crucial role in Sheffield United's rise from League One back to the Premier League, helping Chris Wilder's team achieve a ninth-placed finish in the 2019-20 season.
Fleck made another 26 appearances for Sheffield United in the 2022-23 season en route back to the big time, but he has only played four times in the top flight this season, and his only start saw him last just 45 minutes in the 5-0 hammering at Burnley.
Fleck was under contract at Bramall Lane until the summer, but Blackburn have confirmed that the Scotland international has joined on a free transfer after seven and a half years in a Blades jersey.
More to follow.