Reading travel to the Den on Saturday looking for their first win in five against the London club.
With three former Royals in the Lions' first team squad this season, and links off the field with former Millwall keeper Tony Warner on Paul Ince's coaching staff, there are many who have played for both.
Here are seven from the last two decades.
Darius Henderson
One of Reading's first Academy graduates in 1999, the journeyman striker played almost 100 times for the club at the start of the Noughties, hitting 15 goals in the early years in RG2.
A second and third tier striker for the majority of his career, he joined Millwall in 2011 where he enjoyed a successful two-year stint.
19 goals in his debut campaign, including a strike against Reading on the opening day, he hit just seven the following year and was allowed to join Nottingham Forest before winding his career down with Leyton Orient, Scunthorpe United and Convetry City.
James Henry
Another from the Royals' Academy, Henry played just 18 times in four years with the first team and embarking on five loan spells.
Two temporary spells at the Den preceded a four-year spell on a permanent basis with more than 130 appearances.
An EFL stalwart, long spells with Wolves and Oxford United followed, with the midfielder captaining the U's.
Shaun Cummings
A Championship winner with Reading in 2012, Cummings played almost 90 times in what was a golden period of success in Berkshire.
Displaced by Chris Gunter upon promotion, a three-year stint with Millwall followed- playing regularly upon relegation to League One.
55 appearances with the Lions was followed up with small spells with Rotherham United and Doncaster Rovers before retirement.
Alex Pearce
A Reading legend, and another Academy graduate, the defender was an ever-present during a successful period of Cup runs, Championship titles and Premier League football.
240 appearances in nine first team years, the centre-half joined Derby County and had a 15-match spell on loan at Millwall before making the switch permanent the following summer.
Club captain in London, Pearce played more than 80 times in three years before being released over the summer, now at AFC Wimbledon in League Two.
Jake Cooper
The fourth Academy graduate on this list, giant centre-back Cooper played more than 50 times for his hometown club regular under Steve Clarke.
Discarded by Jaap Stam in 2016/17, he enjoyed a loan switch to Millwall before moving permanently the following summer.
Going from strength-to-strength, Cooper has transformed into one of the top defenders in the league, making more than 250 appearances for the club and captaining the side on many occasions.
A threat in both boxes, Cooper scored the winner the last time these two sides met in Spring.
George Evans
A product of the Manchester City youth system, spells in League One and League Two had prompted Brian McDermott to bring Evans to the Madejski Stadium in July 2016.
71 appearances in two-and-a-half years, including a successful year in which the Royals were one kick from the Premier League, came to an end in 2018 when he moved to Derby County.
Enduring an injury-hit spell at the Rams, he's been a regular for Millwall since arriving in 2020.
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