Pompey are also understood to have tabled a bid for Watford defender Mike Williamson as they look to bolster their squad.
Skysports.com understands a fee has been agreed for Brown and the former Tottenham and Manchester City man has been offered a three-year deal at Portsmouth.
The recent takeover by Sulaiman Al Fahim has cleared the way for the club to make their moves in the transfer market and they have wasted little time in moving for Brown and Williamson.
Portsmouth have already completed the signing of Watford striker Tommy Smith and they are hoping to bring his former Hornets team-mate Williamson to Fratton Park
Williamson has handed in a transfer request at Vicarage Road in a bid to win a move to Portsmouth, but the request has been turned down by Watford.