The 23-year-old from Cameroon began his career with Queens Park Rangers, coming through the academy system at Loftus Road before going on to turn professional with the Rs in 2013.
He moved on loan to Southport to gain experience in his first season as a professional and played seven times for The Sandgrounders.
After a couple of years gaining more experience at senior level, Monthe made the move to Forest Green in November 2016 and played his part in helping the Gloucestershire club to promotion from the National League last season.
This term, the tall and powerful central defender has made twenty appearances for Mark Cooper`s side.
Rovers manager Micky Mellon said: "I’m really pleased to bring Manny into the group.
“With the injury to young Evan Gumbs, we needed to add to the defence and Manny has good quality and experience which will help us at this important stage of the season.
“I’d like to thank Mark Cooper and Forest Green Rovers for allowing him to join us for the rest of the season”.