The central midfielder who has made over 120 league appearances for Saints since arriving from Colchester United in 2009 returns to the Championship just months after earning promotion with Nigel Adkins’ side.
Dean was previously a trainee with the Seagulls and represented them between 2000 and 2008 before joining the U’s in League One.
Hammond subsequently switched to Saints in the third tier and became an instrumental part of the team’s rise into the Premier League.
He last featured for the team in Tuesday evening’s 4-1 Capital One Cup round two win over Stevenage at the Lamex Stadium.
Everyone at Saints wishes Dean well with his year at Brighton.