Monthe, who can play at left-back or centre-half, started his career at QPR but failed to make a senior appearance.
The 21-year-old has also played for Southport, Whitehawk, Hayes and Yeading, and Havant and Waterlooville.
He could make his debut for Rovers when they travel to Wrexham in the National League on 26 November.
Details of the length of his contract at Forest Green have not been disclosed.
National League leaders Forest Green Rovers have signed defender Emmanuel Monthe from National League South side Bath City.