The new deal will keep him at Whaddon Road until the summer of 2016.
Since making his debut at Brighton's Amex Stadium in the Capital One Cup first round, former Birmingham City youngster Deaman has made 13 starts and three substitute appearances for the Robins this term.
Boss Paul Buckle was happy to tie down another of his up and coming players, having already secured Joe Hanks, Zack Kotwica, Omari Sterling-James and Harry Williams on longer contracts in recent weeks.
"Jack can play in a number of positions and he has already filled in at centre-half, full-back and in a defensive midfield role since I've been at the club," Buckle said.
"He is versatile and while there is a lot to work on, he is one we like and are looking forward to seeing develop at Cheltenham Town."
London-born Deaman spent time on loan at Cheltenham from Birmingham during the 2012/13 campaign, without making an appearance.
He left St Andrew's at the end of that season and was set to sign for Torquay United after a successful trial period, but he picked up an ankle injury in training and after regaining fitness he ended up joining Eastbourne Borough.
Enjoying his first taste of senior first team football, Deaman shone for the Conference South club and earned his chance in the Football League this term.
Everyone at Cheltenham Town would like to congratulate Jack on the arrival of his baby daugher Theadorah on New Year's Day.