The 32-year-old striker, who started his career at the City Ground and scored 51 goals in 181 league appearances, has been training with Forest for the past six weeks having spent a spell in China with Guangzhou R&F.
Nottingham Forest boss Steve Cotterill: "We hope he will bring us a few goals.
"He has had a happy knack of scoring a few goals wherever he has been. He brings experience and he brings a good local knowledge."