Boro head to the capital on Saturday as Michael Carrick's men aim to complete a league double over Millwall.
It is sure to be a fierce battle with the play-off-chasing Lions, who we profile below...
The background
Millwall won their most recent two games to climb to ninth in the table, three points short of the play-off positions.
They have also lost just three of their last 10 as they eye a return to the top flight for the first time since 1990.
After a difficult start to the season which saw them win one of their first six matches, it has been another solid Championship campaign for the Lions.
Their most recent victories came over Portsmouth and Sheffield United, who they defeated 1-0 in midweek.
The boss
Alex Neil is at the helm, having been appointed in late December, replacing club legend Neil Harris.
Having been 13th when he took over, Neil has guided Millwall up the table and they are eyeing a late push for the top six.
The Scot has won promotion from the Championship before, having emerged victorious in the play-offs with Norwich City in 2015.
Neil also has a League One promotion on his CV from his time at Sunderland, while he has also managed Hamilton Academical, Preston North End and Stoke City.
Key players
Josh Coburn has been key for Millwall since joining on loan from Boro last August, with his goal against Sheffield United on Tuesday, his fifth for the London club. He will be ineligible to face his parent club.
Former Boro midfielder George Saville brings strong experience to the midfield, as does Duncan Watmore up-front, although he has been out since February through injury.
A number of Millwall players missed out on Tuesday's game due to injury, with Japhet Tanganga and Femi Azeez among them.
Other important performers include left-back Joe Bryan, defender Jake Cooper, midfielder George Honeyman and striker Mihailo Ivanovic.
Past meetings
Boro won the reverse fixture 1-0 at the Riverside in the lead-up to Christmas.
We were also victorious on our last visit to The Den, with Lukas Engel, Isaiah Jones and Marcus Forss on target in a 3-1 win last January.