Marsden linked up with Town at the start of pre-season and has featured in friendlies against TNS, Liverpool and Galatasaray.
The 19-year-old startedhis career at Liverpool and has enjoyed spells with Wigan and Celtic amid periods in non-league football, while he finished last season at Stoke.
"He's a young player with a lot of potential and he's done well in training during his time with us," manager Graham Turner told the club's official website.
"We have seen his touch and goalscoring potential in training and his appreciation of where the goal is.
"It's great to have him on board and we hope that John can fulfil that potential he has."