The 21-year-old defender was a free agent following his release by Birmingham City.
"Jack is here to provide stiff competition for places in defence," manager Mark Yates told the club's official website. "We need a few defenders in the building and he can play in a few different positions so we're delighted to have signed him.
"He is a young lad who we have known for a while when he was with Birmingham and he was perhaps a little unlucky that we didn't take him last year."
Deaman had a four-week loan stint with Cheltenham during the 2012-13 season, but he didn't make an appearance at Whaddon Road.