Purpose <p>In pT1 rectal adenocarcinoma, adverse pathological features guide the indication for radical surgery; however, they are present in only 10–15% of cases. Therefore, in most patients, accurate en bloc local excision with clear margins and precise submucosal invasion assessment is essential for appropriate risk stratification. Beyond absolute depth, a major challenge is the wide interindividual variability in submucosal thickness. The Taulí-pT1 classification, based on the measurement of healthy residual submucosa (hrSB) from the muscularis propria, has been proposed as an objective and reproducible system. This study aimed to validate this classification.</p> Method <p>An interobserver study was conducted on 30 patients with pT1 rectal adenocarcinoma treated by transanal endoscopic surgery. Four pathologists with varying experience levels independently evaluated digitized histological slides, measuring hrSB, total submucosal thickness, and invasion depth. They also classified specimens as sm1, sm2, or sm3. Interobserver agreement was assessed using intraclass correlation coefficients (ICC), and Fleiss’ and Cohen’s kappa indices.</p> Results <p>Excellent interobserver agreement was observed for hrSB (ICC = 0.99; 95% CI: 0.98–0.99), total submucosal thickness (ICC = 0.96; 95% CI: 0.93–0.98), and depth of invasion (ICC = 0.94; 95% CI: 0.9–0.97). The Taulí-pT1 classification demonstrated good agreement (Fleiss’ kappa = 0.71). Identification of the muscularis mucosae showed moderate agreement (kappa = 0.612).</p> Conclusion <p>The Taulí-pT1 classification demonstrates high interobserver reproducibility, even among pathologists with varying levels of experience, supporting its utility as an objective and standardized tool for the assessment of pT1 rectal adenocarcinoma.</p> Clinical trial registration <p>ClinicalTrials.gov Identifier: NCT06218108.</p>