The former Leeds youth team captain has signed a deal until the end of the season after a successful trial period with the club.
The 21-year-old has had short spells in non-league with Harrogate Town and Frickley Athletic, before taking the opportunity to come back into the professional ranks with the Millers.
“He is 6’3”, very athletic and a box-to-box type of player,” boss Neil Redfearn said.
“He will give us a physical presence and he can play so it is a great opportunity for him, he has to grasp that opportunity now and make a career of it.”
Redfearn added: “Simon was one of the group that got first-team exposure when I was at Leeds. He was in the group of Charlie Taylor, Alex Mowatt, Lewis Cook. He was captain of the 18s and 21s and he lost his way for whatever reason and found himself out of it.
“But before I got the Rotherham job, I knew Simon was out there and knew full well that if I got another chance in football he was someone that I would look at because potentially he is as good as the lads that are there and doing well at Leeds.
“We have got a top young player who is young enough to make an impact.
“He is someobody that I had in and around the first team at Leeds. He is more than capable and now he has an opportunity to go and earn something else.
“He is fit and available and capable of playing at Championship level in my opinion but he has got to get his around the newness of it, his team-mates and what we expect from him. But he’s got a head-start.”