The deal between the two clubs is believed to have been between 300,000 and 500,000 with extra bonuses of up to 1million depending on appearences and International caps.
After playing over 100 games for the bees it is a big blow for the club to have lost their two starting centre backs in two weeks, however chairman Greg Dyke said,
"We were duty bound to inform Sam of Reading's interest and are delighted both for the player and with the deal, as had he remained with us until the end of the season he could have left the club for nothing."
The player himself also told journalists,
"It is every player's dream to play in the Premiership, and I am delighted to get this opportunity.
"I have spent some time here in the last few days, met some of the lads and it seems a great club. Everything feels right and I think this move will lead to good things for me."
"It was a tough decision to leave Brentford and I would like to thank them for putting faith in me during my time at the club."