Persistence has finally paid off for the Lions, who have signed the striker in a club-record deal after seeing several earlier bids turned down this summer.
The 26-year-old will initially join the Championship outfit on loan until January when the move will become permanent. Barnsley have inserted a sell-on clause into the deal.
The decision to cash in on Bradshaw marks a departure from the club’s stated desire of retaining their leading players in the summer window.
Reds’ chief executive Gauthier Ganaye has acknowledged that fans will be “disappointed” at the decision, the latest in a long line of sales of leading players.
But he has stressed that the Oakwell hierarchy were forced to make a pragmatic decision after Bradshaw expressed a desire to move south due to personal reasons with the Reds also mindful that the forward would have been out of contract next summer and able to walk away for nothing.
Ganaye said: “We understand that our fans will naturally be disappointed. This is part of football and when a valuation is met that suits all parties the board of directors have a decision to make together, for the long-term stability of the club.
“We were determined to keep Tom at Oakwell, but we were always prepared if he did go and we now want to reinvest the money in the targets identified.”
The Reds are focusing on bringing in a couple of new signings ahead of next Friday’s loan deadline and remain confident of acquiring a like-for-like replacement for Bradshaw.
Portsmouth’s Conor Chaplin remains a target with the club also focusing on another frontline striker to step into the shoes of Bradshaw.
On his new signing, Millwall manager Neil Harris said: “Tom has been our number one transfer target attacking-wise all summer. Unfortunately it became public back in June and July that we were chasing him. We have had to work extremely hard alongside Barnsley, trying to get the deal done. To finally get it completed is really pleasing.”