The 21-year-old left-back joins from Chelsea on a long-term contract for an undisclosed fee after joining the side on loan in January where he made five appearances in all competitions, starting the final three games of the season.
Millwall just missed out on the playoffs last season and Sturge could be the defender to push them further up the table now that he has joined on a permanent basis. The 21-year-old left-back will add depth and Steve Gallen, Millwall's Director of Football, expressed how happy he is to see him join the side.
"We're so pleased to bring Zak in on a permanent basis.
"He came in on loan in January and whilst we knew how good of a player he was, Joe Bryan was playing exceptionally well and therefore Zak was unable to get many opportunities on the pitch.
"When he did come in, though, he was excellent. He played in the last three games of the season and was very good, too. Everyone knows Zak now and how he is a player; he has a great left foot and loves to get forward, but he is also good in one-v-one situations defensively.
"He is also a very good personality around the training ground and is a good signing for the club - it shows the direction that we're going in."
Sturge never made a single league appearance for Chelsea and will be hoping to make a real difference at the Championship side who certainly have the quality to make it to the play-offs next season as they continue to add to their talented squad this summer.