Ledley joins the League Two promotions hopefuls as Wales’ eighth most capped player with 77 appearances for his country.
The 34-year-old has played over 500 games for Cardiff, Celtic, Crystal Palace, Derby and Charlton in a 17-year senior career.
His stint at Celtic from 2010 to 2014 playing under Neil Lennon included three Scottish league titles and a Scottish Cup success.
He captained the side on a number of occassions while standing in for Scott Brown.
Ledley, who had a short spell in Australia earlier this year with Newcastle Jets, told Newport's website: "It's a privilege to be here.
"I was here last January training with the team and the chairman and manager both welcomed me, so to come back and repay them for what they did for me is remarkable.
"It's been great what the team have done so far and we want to carry that on now.”