Melligan, a Republic of Ireland U'21 International, spent most of the 2002/03 season on loan at Kidderminster, and scored 12 goals in 31 starts. He also made 21 appearances on loan at Doncaster Rovers, playing an important role in their Division Three Championship success last sason.
Robins boss John Ward said "JJ is only a young player but he already has a good pedigree at this level," he said.
"I knew of him from my time at Wolves but I've also seen him a number of times for Kidderminster and Doncaster so I know how effective he can be.
"He had a good scoring record from midfield for Kidderminster but Doncaster tended to play him out wide last season.
"I see him more as a central player. He is very busy and lively and works well off the strikers.
"Cheltenham supporters may remember him winning a penalty and scoring from it for Doncaster last season so at least we have stopped him from doing that again!"
JJ's christian names are John James, but he has become more widely known as 'JJ'.
Also today, the Robins were drawn against Colchester United in the Carling Cup. The match will take place at Layer Road, Colchester, in the week commencing 23rd August.