BREAKING: Former Arsenal winger becomes Warnock's first Millers signing
The 32-year-old was most recently with top flight side Crystal Palace, where he worked under Warnock.
Thomas told the club website: "I have been training everyday and building my fitness up.
“The manager and Kevin (Blackwell) have come in and they wanted to get the deal done, I have signed until the end of the season and we all share the same common objective which is to try and keep the club in the Championship.
“Initially it looked as though I was going to the USA or Asia but the opportunity arose to come and train here and to see if both parties were happy.
“I have kept myself fit, the training has been very good and Warney, the fitness coach, has been great with me helping me do extra. I managed to get 90 minutes under my belt in the reserves last week, and I will be looking to get as many minutes in as possible.”
It is also believed that Farrend Rawson is still in contention for the Millers' trip to Burnley this weekend.
The young centre-back played for parent club Derby County's reserves yesterday evening against Blackburn Rovers.
However, we understand that under the terms of his youth loan deal Rawson can still play for the Rams' youth team and has done on a few occasions this season.