The 30-year-old midfielder left Selhurst Park over the summer and Ledley will now return to football in the Championship.
Ledley, who has been capped 73 times by the Welsh national team, grew up in the academy of hometown club Cardiff, before opting for a move to Scottish giants Celtic. Ledley spent four years at Celtic Park, racking up 10 Champions League appearances along the way.
After completing a £3m move back to England in 2014, the midfielder accumulated 96 appearances for the Eagles, scoring six times and assisting another two.
Ledley will join the likes of Tom Lawrence, Andre Wisdom, Tom Huddlestone and Curtis Davies through the doors at Pride Park this summer, Gary Rowett clearly looking to add Premier League experiences to the Rams first team.