Stewart, who can operate at left-back or as a winger, has been a free agent since his release by Notts County.
The 30-year-old's previous clubs include Watford, Derby and Leicester and he has represented England at Under-21 level.
He will stay with the Sky Blues until the end of May.
The deal was completed ahead of the transfer embargo placed on Coventry on Friday and has been ratified by the Football League.
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