Jem Karacan has been granted permission by Reading to remain at Millwall until the end of the season.
The midfielder initially joined the Lions on a one-month loan deal at the end of March, an agreement which is due to expire after Saturday's encounter with Leeds.
He has made three appearances for the League One club and has impressed the management at The Den.
The extension to his loan will now allow him to feature in Millwall's final two games of the season against Carlisle and Swindon.
Kenny Jackett's side need four points from their remaining three fixtures to ensure that they do not suffer a second relegation in the last three seasons.