Williams was a free agent following his release by Barnsley at the end of last season and has quickly found another club, putting pen to paper on a contract which will keep him at New York Stadium until 2019.
The 23-year-old Australian arrived in English football in 2010 and has since played for Portsmouth and Fulham in addition to enjoying loan spells at Gillingham and Oxford United.
"I heard a couple of weeks back that Rotherham were interested through my agent, and as soon as I heard that I knew it would be a move that would appeal to me. I got a call from the manager and that was the real turning point for me because within the first two minutes of the conversation I knew that this is where I wanted to be. It is a good club with passionate fans. The manager knows what he wants, and it is what I want as well, so it is a perfect fit for me," he told the club's official website.
"The manager here didn't sound like the majority of managers, and that's why I was so keen to sign. He is different, he was engaging and I enjoyed talking with him. He has a plan here, and it sounded great and I was fully in agreement with what he had to say. I am really looking forward to playing here and hopefully get back to the Championship because everyone knows that is where we want to be.
"There is no shying away from that, that's what we want to achieve here. There were a few clubs that showed an interest, but as soon as the manager here told me about the plans and my role in that, it made me want to come here. I've had a few injuries, but I've been fit for the past year now so that is all behind me. I am young, fit and hungry for success and I really want to play and that's why I am here."