The midfielder became a free agent at the end of last season when his contract at Crystal Palace expired.
Now, the 30-year-old will be playing football in the Championship after the Rams finalised a transfer this evening.
Ledley, who will be available to make his debut against Birmingham City on Saturday, has agreed to join the club on a short-term deal until January.
Derby is the fourth club that the midfielder can add to his CV following spells at Cardiff City, Celtic and Palace, where he spent the last three years of his career.
The Welshman's new side currently sit 15th in the Championship following three wins, one draw and three defeats.