Professional Footballers' Association chief Gordon Taylor has confirmed the former Portsmouth, Arsenal and Tottenham defender had reached a settlement with County which sees the Meadow Lane club release the player's registration.
Campbell, 35, has left by "mutual consent" just one game into a lucrative five-year deal.
"Sol Campbell is now a free agent," Taylor said. "His contract dispute has been resolved, and he is free to pursue football in the January transfer window. He can sign for any club he wants now."
Chris Hughton : "Sol would be one of a number of players we will be assessing over the next month.
"We have got to make sure that come the end of January, we have got the right squad, we have brought in the right players - and of course, it is not only us that know about Sol's situation.
"He would be one of just a number of players we would be interested in."