Dragonflies and damselflies (Insecta: Odonata) were surveyed in the string fen of the Frontenac National Park, situated in the mixed temperate zone of the Appalachian region of southern Québec (Canada). This fen biotope, considered exceptional due to its ecological uniqueness and southern location, was surveyed 22 times from 2014 to 2016, covering the full flight period of the study taxa. Hundreds of adult specimens and exuvia were collected, which together with behavioral observations of adults, served to rate species abundance and to establish which were resident. The fen hosts a unique community of at least 52 dragonfly and damselfly species (22 common, 11 occasional and 19 accidental). In total, 33 were proven or considered likely to have resident populations. The ranges of 21 species show a clear northern affinity, with some likely being relic populations in the biogeographic context of the fen, such as the Quebec emerald (Somatochlora brevicincta), which until recently, had never been observed south of the St. Lawrence River. Discoveries included 4 threatened or endangered species: the harlequin darner (Gomphaeschna furcilata), ebony boghaunter (Williamsonia fletcheri), Quebec emerald and elfin skimmer (Nannothemis bella); and 3 species that have recently expanded into Québec: the river bluet (Enallagma anna), familiar bluet (E. civile) and painted skimmer (Libellula semifasciata).