Aim: To asses the concordance of histological alterations between appendix and colon in patients with chronic slow-transit constipation (STC) and to determine the feasibility of using the appendix as a full-thickness morphological surrogate for personalizing the extent of colectomy. Materials and Methods: A single-center retrospective pathomorphological study included 89 patients with CSTC refractory to conservative therapy after colectomy with appendectomy. Full-thickness histological examination of the colon and appendix was performed using semiquantitative assessment of the mucosal, submucosal, muscular, neuronal, and interstitial cells of Cajal (ICC) blocks. Immunohistochemical analysis of ICCs was performed using CD117 (c-kit). Histological profiles were categorized into myopathic, Cajal, neuropathic, and histologically intact subtypes. The concordance of histological phenotypes between the appendix and the colon, the correlation of integrated block scores, and the diagnostic efficacy of the appendix were analyzed. Results: Within the distribution of colonic phenotypes, the myopathic variant predominated (51.7%), followed by the Cajal subtype (21.3%), the neuropathic subtype (19.1%), and the histologically intact variant (7.9%). The exact match of the histological subtype between the colon and the appendix was achieved in 98.9% of cases, while the concordance of the dominant phenotype reached 100%. Cohen’s kappa for exact subtype agreement was 0.985, and the weighted kappa was 1.00. The strongest correlations were identified within the ICC block (r = 1.00), the neuronal block (r = 0.96), and the muscular block (r = 0.93) (p < 0.001). For the Cajal subtype, the diagnostic sensitivity, specificity, and overall accuracy of the appendix were 100%. Conclusions: The appendix demonstrates an exceptionally high concordance with the colonic histological phenotype and can be regarded as a phenotype-predicting, full-thickness morphological surrogate in patients with chronic slow-transit constipation