Derby have completed the signing of Dave Martin from Millwall for an undisclosed fee.
The 25-year-old spent the last three months on loan at Pride Park where he impressed manager Nigel Clough enough to sign the player on a a permanent basis.
The winger joined the Lions in 2008 from Crystal Palace, making 75 appearances for the club before moving to the Rams in January, where he impressed, scoring one goal in 11 games in the Championship.
Martin makes the move to Derby despite his former side's promotion to the same division following Kenny Jackett's team's 1-0 victory over Swindon in the League One play-off final at Wembley on Saturday.
"He settled in well during his loan spell and will come back for pre-season with a great deal of confidence because of the way he finished the season," said Clough.
"David is an exciting player, who we hope can develop here. He's hungry to succeed.
"He had to be patient waiting for his chance, but with Stephen Pearson likely to miss the start of the season after undergoing surgery he has the chance to make the position his own from the start."