Bradley made 102 appearances for Walsall however he was released from the League One team at the end of last season.
He told the club’s official website: “I was delighted to get my first cap for Wales. It shows I’ve got a bit of ability and now I want to show that at Rotherham.
“I like to see myself as a good passer of the ball and a box-to-box midfielder. One thing I need to improve on is my goalscoring but I am looking forward to showing what I can do.”
He added: “I want to play as many games as possible and win as many games as possible.
“Promotion is the main ambition this season and I think we can achieve it. I’m really happy to be part of this club.”