The nature conservation depends on the completeness of the knowledge about the biodiversity in the study area. Today, data on distribution and diversity of Diptera taxa remain fragmentary. This paper supplements data on the diversity of Diptera species in Europe, Russia, and the Republic of Mordovia. We have studied 1321 specimens of 241 species collected in 2013–2022. Among them, there are species, which are new for Russia (Neoempheria brevilineata, Desmometopa discipalpis) and for both Russia and Europe (Nepphyllomyza flavescensis, Phyllomyza auriculatusa, P. luteipalpis, P. orbita, P. piceus, P. striolatum). We have found for the first time 231 Diptera species for the fauna of the Republic of Mordovia; the families of Bolitophilidae, Keroplatidae, Scatopsidae, Platypezidae, Micropezidae, Opomyzidae, Asteiidae, and Milichiidae have been found for the first time in this region. Anthrax incomptus (Bombyliidae), being previously reported for the Republic of Mordovia, has been excluded now from the fauna of this region. Nine Diptera species have been previously indicated for the Republic of Mordovia without reliable confirmation by collected specimens. In this study, we have confirmed their presence in the Republic of Mordovia. Taking into account the results of the present paper, the Diptera fauna of the Republic of Mordovia includes currently 1187 species belonging to 71 families. Data on collecting methods have been presented for the found species. We provide a comparison of collecting methods and suggestions for an effective inventory of the regional Diptera fauna. We are of the view that the number of the used collecting methods affects positively the results of the fauna inventory.