Abstract <p>Old-growth forests with very old trees are important ecosystems for nature and humans. There are practically no publications regarding their structure in the Altai Mountains. The objective of the study is to identify old-growth forests in the Altai Mountains using structural and successional criteria of the old-growth stage based on the analysis of the structure of their tree storey. In these forests, 29 sample areas were established in 8 study areas on 4 mountain ranges. Their tree storey is dominated by Siberian pine and Siberian larch. The average maximum age of pine was 410–590 years, larch—360–560 years, the average age of the main generation of trees was equal or less. These uneven-aged stands are morphologically complex, with 1–3 tree storeys and 2–4&#xa0;generations of trees. Ontogenetic spectra of the Siberian pine coenopopulation are complete, while those of the larch are incomplete. Siberian pine dominates among the seedlings and saplings. Age distributions of tree coenopopulations have an inverse <i>J</i>-shaped, uneven, and multimodal form. A distinctive feature of these forests is the presence among the age parcels (<i>gap</i>-mosaics) the parcels with dominance of the <i>g</i>₃ (<i>g</i>₃ + <i>g</i>₃ – <i>ss</i> + <i>g</i>₃ – <i>s</i>) state individuals, with the possibility of their further transition to individuals of other growth forms: “living stump” or “living log”. Due to the latter, it is possible to extend the ontogeny of individual trees by another ≈200 years. The complete ontogeny of a tree with further transition to another growth form can be 700–800 years or more. In the absence of catastrophic impacts, old-growth forests of Siberian pine and larch can exist indefinitely. Criteria for old-growth forests in the Altai Mountains have been clarified.</p>