Millwall Football Club is delighted to confirm the permanent signing of Jake Cooper.
The defender, who spent the second-half of 2016/17 on loan with The Lions, has put pen-to-paper on a three-year deal and joins Millwall for an undisclosed fee.
Cooper, 22, rose through The Royals' Academy set-up and leaves the Berkshire club after 54 appearances. He featured 18 times for Millwall during the aforementioned loan spell last season, scoring twice, as The Lions won promotion to the Championship via the League One Play-Offs.
The giant centre-half, who has played for England's U20 side, successfully completed a medical at Millwall's Calmont Road training base on Friday and could be involved in the squad to face Granada at The Den this Saturday.
Cooper told millwallfc.co.uk: "I'm really pleased to be back here at Millwall and to have secured my long-term future.
"We've had to be patient to get the deal done and I'm relieved everything is now complete and I can focus on my football.
"It was an incredible experience to be a part of what we achieved at Wembley and I can't wait to get stuck in again as part of what is a great group of lads.
"Thankfully I've been training and playing in pre-season games since the start of the summer so I'm fit and raring to go."
Lions boss Neil Harris added: "I'm delighted that we have been able to bring Jake back to the club on a permanent basis.
"I've made no secret of my appreciation and respect for how he performed last season and how we've been so keen to sign him this summer.
"He has signed a long-term contract so he can realise his potential here at Millwall. Jake has a strong personality and is a player who is capable of strong performances. I'm very confident that he can go on to become a top, top centre-half for this club.
"I'd once again like to thank the chairman and the rest of the board for their support in getting this deal over the line."
Cooper will wear the club's number 35 shirt.