Background: Surgical resection is the only curative treatment of gallbladder cancer (GBC). However, the role of oncologic extended resection (OER) in advanced GBC (T3/T4) remains unclear. This study aimed to evaluate the effect of OER in patients with advanced GBC. Methods: This retrospective, multicenter study analyzed 419 patients diagnosed with GBC at 17 institutions across 7 countries between 1997 and 2022. Adjusted logistic regression was used to examine factors affecting R1 resection and lymph node positivity. Survival was assessed using Kaplan-Meier curves and multivariate Cox proportional hazards. Results: Of the cohort, 369 patients with T3 GBC and 50 patients with T4 GBC were identified. Predictors of R1 status for patients with T3/T4 GBC included jaundice before surgery (odds ratio [OR], 3.03 [95% CI, 1.68-5.45]), perineural and/or lymphovascular invasion (OR, 2.43 [95% CI, 1.35-4.39]), adjacent organ resection (OR, 2.05 [95% CI, 1.09-3.85]), overall morbidity (OR, 1.64 [95% CI, 1.01-2.66]), and lymph node metastasis (OR, 2.69 [95% CI, 1.55-4.66]). Morbidity was higher in patients (64.8%) with T4 GBC than in patients (38.2%) with T3 GBC, with severe morbidity at 46.3% and 17.1%, respectively. Of note, 90-day mortality was 4.1% for patients with T3 GBC and 12% for patients with T4 GBC. The 3-year overall survival rate was 33% for patients with T3 GBC and 4% for patients with T4 GBC (log-rank P <.001). Conclusion: Advanced-stage GBC outcomes vary with resection status. OER is associated with increased morbidity, particularly in patients with T4 GBC, for whom survival benefits are limited. Careful patient selection for aggressive surgical treatment is crucial to avoid unnecessary morbidity while carefully weighing the potential survival benefits.