Ryan Smith will stay at League One Millwall until 5 May after extending his loan spell from Derby County. The 20-year-old midfielder arrived at the New Den on 21 March and has made four appearances, starting three times.
He was due to return to promotion-chasing Derby after Saturday's game against Huddersfield.
However, the former Arsenal trainee will now be available to play for Millwall until the last game of the season against Bradford.