In the framework of the project “Classification of Vegetation of the Russian Federation” (Plugatar et al., 2020; Kipriyanova et al., 2021), a revision of the modern coenotic diversity of Russia, including aquatic vegetation, is being carried out. It turned out that the types of some plant communities have not yet been described, including the association with the dominance of the Ranunculus mongolicus (Krylov) Serg. Ranunculus mongolicus was first reported in the Flora of West Siberia (Flora..., 1931) by P. N. Krylov as a subspecies of the water buttercup R. aquatilis ssp. mongolicus Kryl. (Krylov, 1931), L. P. Sergievskaya raised the taxon rank to species (Sergievskaya, 1964). In the modern monographic treatment of aquatic buttercups, this taxon is given at the species rank (Wiegleb et al., 2017). It indicates that Ranunculus mongolicus is habitually similar to R. aquatilis, although smaller in all parts. Lamellar leaves are deeply dissected, peduncles are long, all parts of the plant are densely hairy. In arid regions, it behaves like an annual (Wiegleb et al., 2017). In the course of expedition work in 2020 in the Ulagan district of the Republic of Altai, five geobotanical descriptions of communities dominated by the Mongolian buttercup (see Table) were performed, and it was decided to describe a new association. The classification was carried out on the principles of the ecological-floristic approach of J. Braun-Blanquet in accordance with the Code of Phytosociological Nomenclature (Theurillat et al., 2021). Ass. Ranunculetum mongolici ass. nov. unites the communities dominated by the Mongolian buttercup (Fig. 1, 2). The diagnostic species is Ranunculus mongolicus (= Batrachium mongolicum (Krylov) Krecz.). Nomenclature type of association (holotypus hoc loco) — relevé No 5 in the Table. Republic of Altai, Ulagan district, vicinity of the mouth of the Chulyshman River, the oxbow lake of the Chulyshman River, 21.08.2020, 51.33130 N 87.73750 E. Author — L. M. Kipriyanova. We attributed the new association to the alliance Batrachion aquatilis Passarge 1964 of the order Callitricho hamulatae–Ranunculetalia aquatilis Passarge ex Theurillat in Theurillat et al. 2015 of the class Potamogetonetea Klika in Klika et Novák 1941. The range of the community apparently coincides with the range of the species. In Asia, R. mongolicus is found in Siberia, the Russian Far East, Kyrgyzstan, Afghanistan, Mongolia, China, and Nepal (Wiegleb et al., 2017). It is found along the west coast of North America where it is confused with R. aquatilis (as R. aquatilis var. aquatilis and R. aquatilis var. hispidulus); likewise, most South American “R. aquatilis” refers to R. mongolicus. (Wiegleb et al., 2017). In Eastern Siberia, R. mongolicus is rare and does not form communities (Chepinoga, 2015). In addition to the Mongolian buttercup communities from Ulagan district, there are publicly available photographs of cenoses with R. mongolicus taken by A. I. Pyak in the Kosh-Agach district of the Republic of Altai (valley of the Kyzylchin (Buguzun) River, 49.992579° N, 89.088472° E, 15.07.2007, https://www.inaturalist.org/observations/99636689) (see Fig. 3). There are also photos of Yu. O. Kopylov-Guskov with thickets of Mongolian buttercup from the Republic of Tuva (Mongun-Taiginskiy district, 50.180458° N, 90.136025° E, 15.07.2016, https://www.inaturalist.org/observations/65270151) (see Fig. 4), such as A. L. Ebel photo from the Krasnoyarsk Territory (Kuraginskiy district, near the village of Kuragino, muddy bank of a channel of the Tuba River. 18.07.2018, 53.85932° N, 92.60829° E, https://www.plantarium.ru/page/image/id/719518.html) (see Fig. 5) In addition, R. mongolicus forms communities on the Taimyr Peninsula (Krasnoyarsk Territory, Khatanga River valley, Malaya Balakhnya oxbow lake, 22.08.2001, http://byrranga.ru/ranunculaceae/batrachium_aquatile/index.htm) (see Fig. 6). Thus, at the moment it is possible to speak about the communities of the Mongolian buttercup from the Republic of Altai, the Republic of Tuva and the Krasnoyarsk Territory. In the Republic of Altai, association communities are confined to flooded ecotopes, mainly with mineral (clay and sandy) silty substrates and depths from 20 to 60 cm in oligomesotrophic and mesotrophic cold-water reservoirs in the low-mountain and high-mountain belts of Altai, which makes it possible to attribute the association to the alliance Potamogetonion graminei Westhoff et Den Held 1969 combining the vegetation of rooted macrophytes of nutrient-poor (oligo-mesotrophic, sometimes dystrophic) shallow-water fresh water bodies of mountainous regions (Westhoff and Den Held, 1969; Mucina et al., 2016). However, the features of the ecology of communities throughout all the distribution area are not known yet, so for the time being we leave it in the alliance Batrachion aquatilis of the order Callitricho hamulatae–Ranunculetalia aquatilis.