The striker spent the second half of last season at the City Ground on a short-term deal, scoring twice in 11 Championship appearances.
And Forest manager Billy Davies saw enough to convince him Henderson is worth a longer-term investment.
Davies told the club website: "I am absolutely delighted to have Darius on board. He showed us what he is capable of in his spell with us last season and his vast experience will be invaluable to us going into the new campaign.
"To have landed such a key target at this stage in the summer is a huge bonus and it should give everyone great confidence for the future."
Henderson, who played in the Premier League with Watford in 2006/7, said: "I definitely want to get back into the Premier League and, having signed for Forest for another two years, that is my target.
"I believe we have a very strong chance of finishing in the top six next season and maybe, with a bit of luck, we could even claim a top two place.
"That would be great, but it will be difficult. There are some massive clubs with huge potential in this division but I am looking forward to the challenge.
"I feel as fit and fresh as I've ever done and now I will be able to settle and fully concentrate on the task ahead."