The 26-year-old former Dagenham, Cheltenham and Crawley striker signed a contract until the end of the season when joining the Millers from Bournemouth for an undisclosed fee on Tuesday.
"I'm delighted to sign," Thomas told the club's official website. "It's a good club, they're doing well in the league and I'm just looking forward to getting started and hopefully getting some goals."
He added: "Obviously staying longer is a possibility, but in the short-term I'm just happy to be here and I want to get started.
"Hopefully I can impress and help the team get promoted. I've had a quick look at the stadium. It looks like a great place to play football and hopefully I can score a lot of goals here."
- Sports & Recreation