The midfielder, 29, had a loan spell at Fratton Park last term, but following his release from Millwall he signs as a free agent on a one-month contract.
He told the club's official website: "I feel happy to be back and I'm hungry to get out onto the pitch as soon as possible.
"I've been training by myself over the summer, but I feel fit and can't wait to get started.
"I know what to expect from the fans. They do a fantastic job and it's because of them that the club was saved."