Marshall arrived in Europe in 2005 and played for Leeds before moving to the Liberty Stadium based Welsh region where he reached a Heineken Cup quarter-final and won a Magners League EDF Energy Cup titles.
Montpellier head coach Didier Nourault believes that Marshall's international experience will help bring on his side.
"We were looking for players like Justin. He's a high level player with lot of experience and I'm sure he will bring a lot to the team and help our young scrum-halves to improve." said the Montpellier coach.