The tiny creature – which has distinctive fluorescent green legs and mouthparts – has only been recorded at three other places in the UK.
An ecologist identified the spider during the “bio-blitz” event at Wester Moss.
It is also found at Flanders Moss and Ochtertyre Moss, both near Stirling, and in a raised bog near Aberystwyth.
Known officially as Heliophanus dampfi, the spider was first discovered in the UK in September 1989.
Caledonian Conservation’s Chris Cathrine, who confirmed the latest sighting, said the spider’s rareness was down to its preference for raised bogs – a disappearing habitat in the UK.
“It could be that it’s under-recorded because there are very few people that look for spiders in the UK, but it’s associated with bogs that are in good condition so it’s probably restricted to those areas,” he said.
“They’re about 3mm long and they’ve got these fantastic iridescent green palps that catch the sunlight nicely.”
Palps are the appendages found near a spider’s mouth.
Stirling Council said more than 50 people had attended the recent open day at Wester Moss and Fallin reserves, in a bid to record as many species as possible over two days.
Countryside ranger Jennifer Davidson said: “This is a brilliant find and to know there are so few recorded sightings in our country makes it even more special.”