Sullivan, who signed a two-year deal with the Addicks in May 2011, replaces his namesake Neil Sullivan at Kingsmeadow after the veteran was recalled from his loan spell by Doncaster.
Dons boss Neal Ardley told the club's official website: "John Sullivan has been on our radar for a while now.
"We have looked at a number of players in various positions in case we had to act swiftly. This has certainly helped in terms of the goalkeeping situation because we never had to start from scratch."
The 25-year-old has made a total of seven appearances for Charlton and he has previously had loan spells at Colchester and Yeovil.