The actor appeared in the 2010 action movie and co-wrote the script with David Callaham. However, writer Marcus Webb filed a lawsuit alleging copyright infringement at Manhattan federal court on Tuesday, Reuters Canada reports.
Webb claims that The Expendables' screenplay is "strikingly similar and in some places identical" to a script which he circulated for consideration by movie bosses between 2006 and 2009.
He testifies that his work The Cordoba Caper features a group of elite mercenaries hired to overthrow a military dictator - also the centre of The Expendables' plot.
Webb is seeking unspecified damages and also wants an injunction to prevent further copyright infringement in any sequels.
Stallone's spokesperson declined to comment and a lawyer for studio Millennium Films only confirmed that the company had not been served with any legal documents.
The Expendables' distributor Lionsgate has not yet commented on the allegations.
A sequel to The Expendables is currently in production for release on August 17, 2012.
Watch a trailer for The Expendables below: