An in-depth survey of dragonflies and damselflies (Insecta: Odonata) was conducted in 2012 and 2013 at the lac des Atocas, a protected wetland within the Mont-Saint-Bruno National Park, located in the temperate deciduous bioclimatic zone of southern Québec. This small body of water, with peaty banks, covers an area of approximately 0.68 ha, and was used by 53 species of odonates, including more than 40 that live and complete their life cycle there. One of the highlights of the present study was the discovery of a population of the spatterdock darner (Rhionaeschna mutata) (Anisoptera: Aeshnidae), which is a first for the province of Québec, and represents a northern extension of its known range. In addition, several of the other specimens identified were of species rarely reported in the province, and the harlequin darner (Gomphaeschna furcillata) and the slaty skimmer (Libellula incesta) were new additions to the list for the Montérégie region. Finally, observations of exuviae and adults extended the known spring and fall flight periods for a dozen species.