The Robins, who also secured a loan extension for Michael Hector, will keep the 19-year-old centre back until the end of the season, including the play-offs if necessary.
Deaman, who joined the Blues in the summer of 2011 from Wrexham's academy, trained with his new team-mates on Thursday and is available to face Port Vale.
"I've known Jack for a couple of years and he is a good player and athlete," manager Mark Yates told the club's official website.
"We need to cover every eventuality and we are a little light in defence. Michael Hector is close to a suspension so Jack will give us some protection. He knows what to expect here and he is ready to try and prove himself."