The discovery of a population of the eastern red damsel (Amphiagrion saucium) in the Parc de la Rivière-du-Moulin, an urban park in the city of Saguenay (Québec, Canada), represents a new addition to the known Odonata fauna of the lowland enclave of the Saguenay–Lac-Saint-Jean region. This population occupies a series of spring-fed marshes along the bed of an old meander exposed following a landslide during the Saguenay flood in 1996. Adults specifically use pools dominated by variegated scouring-rush (Equisetum variegatum). Their number declines rapidly from mid-July, which coincides with the start of the breeding season of the northern spreadwing (Lestes disjunctus) and white-faced meadowhawk (Sympetrum obtrusum). At the northern limit of its range, the presence of beaver activity likely plays an important role in the population dynamics of the eastern red damsel. In Québec and other inhabited regions of eastern North America, its specialized habitat, its low dispersal rate and its sensitivity to anthropogenic threats make this species vulnerable. In urban areas, it could benefit from improved stormwater management incorporating holding ponds. The abundance and phenology of all 15 species of Odonata found breeding in the study area in 2020 and 2021 are provided.