Background and purpose: Index cholecystectomy is insufficient for curing T3 incidental gallbladder cancer (IGC), and once residual cancer (RC) is found, the prognosis is often poor. The purpose of this study was to investigate the effect of RC on the prognosis and the optimal choice of adjuvant therapy for R0 reresection patients with T3 IGC. Methods: We retrospectively reviewed data from patients with T3 IGC who underwent radical reresection from January 2013 to December 2018. RC was defined as histologically proven cancer at reresection. Demographics and tumour treatment-related variables were analysed in correlation with RC and survival. Adjuvant (Adj) chemoradiotherapy (CRT) was correlated with overall survival (OS) and disease-free survival (DFS). Results: Of the 167 patients with IGC who underwent surgery, 102 underwent radical extended resection. Thirty-two (31.4%) RCs were found. Hepatic side tumours (T3h) and both side tumours (T3h + T3p) were associated with the presence of RC. In multivariate analysis, RC and lymph node metastasis were independent prognostic factors for DFS and OS (P < 0.05). RC was associated with a significantly shorter median OS (20 vs. 53 months; P < 0.01) and DFS (11 vs. 40 months; P < 0.001) despite R0 resection. For R0 reresection patients with RC and/or lymph node metastasis, Adj CRT significantly improved OS (P = 0.024). Conclusion: Residual cancer and lymphatic metastasis are important factors for the poor prognosis of T3 IGC despite R0 resection, and these patients should actively receive adjuvant therapy.