Adam Clayton joined Blues on 1 September 2020 on a free transfer having left fellow Championship side Middlesbrough in June.
The experienced midfielder penned a two-year contract, with a year's option.
His move to the St. Andrew's Trillion Trophy Stadium saw him back working under Blues Head Coach Aitor Karanka, who signed him during his time as Boro boss.
Clayton left The Riverside Stadium having made 241 appearances - 34 of which were in the Premier League - and scoring one goal.
Born in Manchester, the former England Under-20 international came through the ranks at Manchester City but left to join Leeds United in 2010 before making his City bow.
He spent two years with The Whites where he appeared 50 times and bagged six goals before being sold to Huddersfield Town in 2012.
Between 2012 and 2014 he turned out 94 times for The Terriers, scoring 12 times.
In August 2014, he joined Middlesbrough for a reported fee of £1.5m and went on to become a key player in the heart of the midfield as Boro went on to win promotion to the Premier League at the conclusion of the 2015/16 campaign.