The Chelsea centre-half returns to the Club after a successful loan spell in the previous campaign.
Clarke-Salter, 23, was much sought after by a number of sides during the transfer window but Blues have succeeded in another coup by bringing him back.
He becomes Head Coach Aitor Karanka's 11th signing for the First Team and brings more quality to the Blues backline.
Clarke-Salter made 23 appearances for Blues in 2019/20 and scored once.
After biding his time before an opening arose, he grasped his chance in the team and became a key performer.
When football resumed after lockdown Clarke-Salter was superb in the 0-0 draw at West Bromwich Albion but unfortunately missed the last four Blues matches due to injury.