Human-made wetlands play an important role in maintaining biodiversity and provide a wealth of ecosystem services. In 1996, abandoned farmland near Neuville (Québec, Canada) was flooded to create what is now the marais Léon-Provancher. In 2016, the water quality of this artificially created marsh was analyzed, and a biomonitoring approach using diatoms as environmental indicators was used to assess the state of the aquatic environment. A paleolimnological study of the diatom assemblages in a sediment core taken from the wetland revealed that during its first 20 years, the wetland was characterized by a dominance of benthic taxa preferring fresh to slightly brackish water with a neutral to slightly alkaline pH, and tolerant to fluctuations in water temperature, conductivity and trophic state. Use of the Eastern Canada Diatom Index (ECDI) indicated that the water quality of the wetland was good to excellent during the period studied. The small changes observed in diatom assemblages since 1996 appear to be more correlated to variations in water level than to water quality. This baseline environmental information will be useful for future monitoring and management of the marsh ecosystem, including its catchment, to help ensure its preservation.