The goalkeeper arrives from Chesterfield United.
Aidy White was released, facilitating his move to Barnsley.
Whilst the club have also snapped up a former Redfearn player at Leeds.
The 23-year-old was released by Leeds United in the summer and went on to join the Spireites.
However he has not made an impact at the League One outfit and never played a match for the club.
Despite that Rotherham boss Redfearn knows his talents from his time at Leeds and admits he is surprised the club let him go in the summer.
He is quoted by the Yorkshire Post as stating:
“He is someone I know from Leeds United, I was surprised when they let him go.
“Alex was number two to Paddy Kenny at Leeds, he is a young goalkeeper, a very strong boy with a strong character.
“He is coming to compete for a place."
Cairns will now do battle with Lee Camp and Adam Collin for the first team spot at The New York Stadium as Rotherham battle for survival in The Championship.
Cairns, who has not played a Football League match since a 5-0 loss for Leeds v Blackpool in November 2011, will hope he has what it takes to prove his worth this time around.