Despite the latest 8th edition American Joint Committee on Cancer Staging Manual guidelines, disagreement still exists among pathologists regarding staging deeply invasive colonic adenocarcinomas ≤1 mm to the serosal surface. In this retrospective study, 151 untreated colonic adenocarcinomas staged initially as either pT3 or pT4a and with available 5-year follow-up data were retrieved and re-categorized: Group 1 (38 cases): pT4a with tumor at the serosa; Group 2 (49 cases): tumor ≤1 mm from the serosa, with intervening reactive fibrosis (40/49) or inflammation (9/49); Group 3 (64 cases): pT3 tumor >1 mm from the serosa. Clinical outcomes were analyzed. Groups 1 and 2 tumors showed significantly lower 5-year recurrence-free survival and lower overall survival rates (log-rank p < 0.001 for both), when compared with Group 3 tumors. Even after adjusting for adjuvant therapy and nodal metastases, the proportional hazards ratios for the risk of death (p < 0.001) and risk of recurrence (p = 0.005) showed significantly higher risk in Groups 1 and 2 compared with Group 3. The synchronous nodal (p = 0.012) and metachronous distant metastases (p = 0.004) were also significantly more in Groups 1 and 2 versus Group 3. Colonic adenocarcinomas ≤1 mm from the serosal surface behaved more akin to “bona fide” pT4a tumors at the serosal surface in our study with regards to clinical outcomes. We recommend these tumors be staged as pT4a rather than pT3, as supported by outcome data in our study. We hope this will also ensure reproducibility and consistency in staging these tumors across institutions.