oper is currently on loan at National League outfit Gateshead for the rest of this season, but he has done enough for the Millers to give him a 2 1/2 year deal.
The 19-year-old has made three appearances for the Millers this season, all in the Football League Trophy.
"I'm really happy to have signed a new contract. Coming into this season with only a one-year deal, I had a point to prove in order to earn a fresh one and I've managed to do that which is brilliant." Cooper told themillers.co.uk
"After pre-season I came back strong so the gaffer and his staff gave me a chance in cup games which I appreciate, but it was good to get out on loan and get some more games under my belt."
"I'm really enjoying my time up at Gateshead and it's been a learning curve for me, having to adapt to men's football. Words can't describe how valuable the experience is, but I'm still only young with a lot to learn."
"It's great to know that the staff at Rotherham trust me, and whilst I've been away I've been in regular contact with Matt Hamshaw. The manager will occasionally message me too which is always nice, knowing he's looking at my development."
"I would love to play in the Football League, so I'll just keep my head down and continue working hard."