Shaw, who was told he can leave Bramall Lane a month ago has failed to impress since his signing from the Gills last January, he netted on his debut against West Ham United in a 3-3 draw and has failed to be given match action.
Paul Shaw
Shaw told Rotherham's official website: "I am pleased to get the chance to come to Rotherham and play as much football as possible. I have played against them several times so I know plenty about them, they are always a hard working side.
"Things didn't quite work out for me at Bramall Lane so this is a good move and I will consider making it a permanent one, I am looking forward to making my debut but it is up to the manager as to whether I play on Saturday."