BACKGROUND: A unified system for assessing the severity of congenital clubfoot in newborns and young children worldwide remains to be established. “Rigidity” of foot deformity refers to the degree of “resistance” of foot tissues during manual correction of elements of the deformity and is often used in subjective severity of foot deformity assessment. However, there is no objective quantitative assessment for the degree of foot rigidity. AIM: The study aimed to introduce a novel clinical sign — ”rigidity of foot deformation”, which enables objective assessment of the severity of foot deformity in congenital clubfoot. MATERIALS AND METHODS: Before applying the first plaster cast, a clinical dynamometric examination was performed on 350 feet of 229 children, followed by a mathematical calculation of the foot rigidity index. Statistical analysis was performed using the nonparametric Mann–Whitney U-test and Spearman’s rank correlation coefficient. Differences were considered significant at p 0.05. RESULTS: Significant differences were found in all clinical and dynamometric parameters between congenital clubfoot of I–II, III, and IV degrees (p ≤ 0.05). Generally, the higher the degree of deformity, the more effort required to eliminate it, the smaller angle of simultaneous correction, and the higher index of foot rigidity. The results of Spearman’s correlation analysis of clinical dynamometric examination indicators in children with congenital clubfoot of I–II degree may indicate the mobile nature of the foot deformity; III degree, a rigid version of the deformity; and IV degree, an extremely rigid degree of deformity. CONCLUSIONS: Rigidity of the foot deformity is a crucial clinical sign that characterizes the severity of the foot deformity, which has a quantitative characteristic — the rigidity index. Initial data on foot rigidity enables objective assessment of the severity of the deformity and selecting an individual approach to its elimination when applying staged plaster casts using the Ponseti method.