The 30-year-old defender has signed a two-and-a-half year deal with Forest, having spent the last ten years at Vitality Stadium.
After completing his move to The City Ground, Cook said: "You can see the progress the club is making and I'm excited for the new challenge.
"I thought it was the perfect time in my career to make this move to hopefully come and contribute and help get this club back to where it wants to be.
"The history of of the club speaks for itself and I know how passionate the fans are. I've played at The City Ground in the past and it's always been electric so now I'm just excited to get started."
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