The biodiversity conservation is based on our knowledge of the complete composition of plants and animals within a certain area. In this paper, we present data of biodiversity research in the Republic of Mordovia (European Russia). The obtained results are reflected in six vascular plant found for the first time in the studied region. Of them, four alien taxa (Philadelphus coronarius, Rosa x majorugosa, Spiraea japonica, Syringa josikaea) are known only in the Mordovia State Nature Reserve now. This underlines the need paying our attention to alien plants that escape into the wild (naturalize) in the least disturbed ecosystems of this Protected Area. The Urtica galeopsiifolia distribution needs to be clarified in more details because of its similarity to the widely known Urtica dioica. Thus, a majority of the species under discussion are alien plants considered as cultivated plants escaping in the wild. Taking into account the newly revealed species, the vascular plant flora of the Republic of Mordovia currently includes 1,472 species. In this study, we propose the continuing the study of man-made habitats in terms of accidentally or intentionally introduced alien species, as well as the searching of hidden taxa, such as Urtica galeopsiifolia found in this study.