The full-back initially moves to the City Ground on loan until the end of the season, with the move set to become permanent in the summer.
Fox spent two-and-a-half years at St Mary’s, having moved from Burnley shortly after the start of the 2011/12 season.
The Scotland international made 43 appearances in his first campaign on the south coast as Saints won promotion to the Premier League, where he had spent time with his previous club.
He started as Southampton’s first-choice left-back upon our return to the top flight, scoring his first goal for the Club in a heavy defeat at Arsenal in September 2012.
However, the emergence of Luke Shaw meant opportunities became more and more limited for Fox as the season went on, although he did rack up 20 appearances over the course of the campaign as Saints secured their Premier League status for the following season.
Starts have been few and far between for the left-back in the 2013/14 campaign, and he now departs St Mary’s having only played six times this season.
Fox now heads to the City Ground, where he will compete with his former Saints teammate Dan Harding for a place in the Forest side.
The 27-year-old made a total of 69 appearances for Saints during his time here, with that goal at the Emirates Stadium his only for the Club.
Danny was an extremely popular member of the dressing room, and leaves with the Club’s best wishes for his future career. He will be fondly remembered for the role he played in helping Southampton return to the Premier League, and will always be warmly welcomed back to St Mary’s.