The 31-year-old has signed a deal to the end of the season and is set to make his debut against another one of his former clubs, Watford, on Saturday.
Henderson is Forest's third signing of the January transfer window, following the loan arrivals of defenders Elliott Ward and Gonzalo Jara Reyes.
He started his career with Reading and has also played for Brighton (loan), Gillingham, Swindon (loan) and Sheffield United.
Forest chairman Fawaz Al Hasawi said: "We are delighted to have brought a player of Darius' experience to the Club.
"He knows all about the Championship and has also played at the highest level in England as well and I've no doubts he will be a great addition to Alex McLeish's squad."