The 28-year-old left-back signed a three-year deal and joins Adlene Guedioura, Danny Collins and Greg Halford, who all completed moves this week.
"It's brilliant, I'm really pleased," Harding told the club's official website.
"I spoke to the gaffer Sean O'Driscoll and liked what he had to say. I'm really excited."
Harding made 25 appearances for Southampton last season as they secured promotion to the Premier League.
Kuwaiti-based Forest chairman Omar Al Hasawi said: "We're delighted with the way things are taking shape.
"Like Adlene, Danny and Greg, Dan is an experienced performer and knows what it takes to gain promotion from the Championship."
Fawaz Al Hasawi, a member of one of Kuwait's richest families, bought Forest, now in English football's second-tier Championship, with the assistance of his brothers earlier this month.
The new owners immediately replaced sacked manager Steve Cotterill with former first team coach O'Driscoll.
Source: AFP
Source: AFP