The purpose of this study is to consider patterns of variability of qualitative traits and phenotypes of Populus × sibirica in Siberia based on data on their variability in certain areas of the region. The study is aimed at summarizing the results obtained on the diversity of qualitative morphological traits of the species, its phenes, assessing intrapopulation and interpopulation variability in phenotypes. To study the variability, 180 trees were examined, on which 2700 leaves were measured in the green spaces of the cities of Novokuznetsk, Novosibirsk, Berdsk, Irkutsk, Bratsk and Krasnoyarsk. When analyzing the qualitative features, the following morphological markers were chosen: the shape of the leaf blade, its apex and base, the presence of basal glands, and petiole pubescence. The phenotype of an individual was determined by a combination of morphological qualitative traits. According to the results of the research, four phenotypes were found in P. × sibirica, common in all stands. In the studied cities of Siberia, individuals with pubescent petioles (46.6–73.0%), an ovoid leaf blade with a rounded wedge-shaped base and a pointed apex prevailed. An assessment of the intrapopulation diversity of phenotypes according to the Zhivotovsky index showed that the obtained indicators are very close. The share of variability in terms of quality traits and phenotypes within the plantation showed a sharp predominance over interpopulation and low differentiation of the studied plantations, both in terms of qualitative traits (3.5%) and phenotypes (2.8%), which once again indicates the cultural occurrence of the taxon. This fact is also confirmed by the extremely high phenotypic similarity of the studied samples (r = 94.2–99.8%) and the absence of their significant differences in phenotypic composition.