The Ob River basin is the last large fragment of the range of native Euro-Siberian poplar (Populus nigra, Populus alba) forests, where these forests remain intact. The taiga section of the Ob River is occupied by association Anemonidio dichotomi-Populetum nigrae Taran 1993, the subtaiga, forest-steppe and steppe sections are occupied by the Equiseto hyemalis-Populetum nigrae Taran 1997. The purpose of the article is to find out syntaxonomic status of poplar forests growing at the junction of the southern taiga and subtaiga subzones and to give them a detailed bryofloristic characteristic. Based on the Braun-Blanquet approach, we studied poplar forests located near the south border of the Ob River southern taiga section, Krivosheinsky district, Tomsk Region, Russia, surroundings of the research station Kaybasovo, 57º14'44"N, 84º11'05"E. In the forests, average species saturation by vascular plant species is 32.3 species per 100 m², its range is 13–56 species/100 m². Moss flora includes 51 species. Average species saturation by mosses is 15.4 species per 100 m², its range is 10–21 species/100 m². Based on new data, total moss flora of the Ob poplar forests has increased from 73 species to 86 species and 1 variety (by 19 %), moss flora of the poplar forests of the taiga zone – from 59 species to 81 species and 1 variety (by 39 %). We identify Kaybasovo poplar forests as new variant Stellaria bungeana of subassociation Equiseto hyemalis-Populetum nigrae betuletosum pubescentis Taran 1997 (Equiseto hyemalis-Populion nigrae Taran 1997, Salicetalia purpureae Moor 1958, Salicetea purpureae Moor 1958).