The wide distribution of Populus × sibirica G.V. Krylov & G.V. Grig. ex A.K. Skvortsov in the Siberian region led to its numerous contacts with plantations of P. nigra L. and the facts of anthropogenic hybridization occurring both in disturbed (ruderal) and natural (floodplain) habitats. Hybrids growing in these territories are distinguished by a significant phenotypic diversity, which makes their identification quite difficult. The aim of this work was to study the anthropogenic hybridization of Populus × sibirica with P. nigra in natural habitats. At the same time, the following tasks were solved: to identify diagnostic morphological features of hybrids; peculiarities of variability of traits in the leaves of the studied taxa; to identify the influence of environmental conditions on the phenotypic diversity of hybrid populations. To achieve the set goals and objectives, 30 reproductively mature trees of P. × sibirica, P. nigra and their hybrids were examined at the mouth of the Biya River and in the city of Biysk (Altai Territory). 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 most important morphological features that make it possible to identify hybrids are the differentiation of short crown shoots and the presence of basal glands. The phenotypic diversity of P. nigra and P. × sibirica plantations is not high, and its growth is observed in hybrids under these conditions. The observed anthropogenic hybridization in natural habitats is asymmetric, which, like the phenotypic composition of hybrid populations, is determined by the action of stabilizing selection.