Millwall have landed Dave Martin from Crystal Palace for an initial fee of £50,000.
Martin becomes Kenny Jackett's seventh signing of the January transfer window and arrives on a deal that could rise to £100,000 - depending on appearances.
Millwall expressed their interest in the 22-year-old earlier this month, and have now secured their man on a two-and-a-half year contract.
Jackett told the club's official website: "David will give the team pace and balance as a wide left-sided player and I expect him to go straight into the squad for Saturday's game against Doncaster."
Meanwhile, Stuart Green and Charlie Sheringham have had their contracts terminated by the club.