The 23-year-old goalkeeper had previously stated he was keen to stay at Roots Hall, where he kept eight clean sheets in 15 appearances to help the Shrimpers gain a play-off place.
But further talks have taken place between Belford and boss Paul Sturrock and no agreement has been struck.
Belford told the Echo: "I can officially confirm that I sadly won't be joining Southend next season.
"Southend will always hold a special place in my heart and I wish the club all the very best for next season and hope they get the promotion they fully deserve."
