New Nottingham Forest manager Steve McClaren had talked to the out of contract striker about a new deal at the City Ground but the lure of returning to the club where he made his name was too much for Earnshaw to ignore.
"I'm obviously sad to leave Forest," he told BBC Radio Nottingham.
"It's a tough decision because the fans just took to me as one of their own and I'm grateful because of that and that's why it's said to leave, but I'm leaving for a good place as well."
Experienced Dutch midfielder Bolo Zenden is being linked with the Reds after the expiry of his Sunderland deal.