The towering defender will spend the remainder of 2016/17 at The Den.
Standing at 6'7", Cooper has already amassed 30 appearances at Championship level despite his young age of 22.
An England U18 and U20 international, the centre-back played as key role in the Reading Academy side which reached the Elite League Final in 2012/13.
Given his stature, Cooper is a threat in both boxes and his four senior goals to date have all come from headers following corners.
He made his Millwall debut against Bradford City on January 21st and will relish the opportunity to gain further experience during his time with The Lions.