Johnson has signed a one-year contract after he was released by Sheffield Wednesday, and Coventry boss Steven Pressley believes that the experience of the 26-year-old can help the younger members of the Sky Blues squad.
The Scot told the club's official website: "Reda is a big, strong, physical centre-half with an imposing presence that also carries a goal threat so I'm delighted that he has decided to join us.
"League One is very difficult one to get out of but Reda has experience of doing just that. He knows the division and he knows what it takes to be successful in it.
"He also brings a level of experience that our younger players can benefit from. He leads from the front, he puts his body on the line and he's a person that can have a positive influence in the dressing room."