BACKGROUND AND OBJECTIVES: The association between extent of resection (EOR) and outcomes after brain metastasis (BM) surgery remains unclear. The aim of this study was to examine the relation between EOR and overall survival (OS)/intracranial progression-free survival (IC-PFS) in this patient group. METHODS: We included patients who underwent BM resection and had postoperative MRI <72 hours. Presence of any residual on MRI was defined as subtotal resection (STR) as opposed to gross total resection. Multivariable analyses were adjusted for known confounders. Post hoc analysis explored the effect size across different subgroups. A secondary outcome was the occurrence of leptomeningeal disease (LMD). RESULTS: We included 867 patients; median age was 61 years (IQR 53-68), and median BM diameter was 3.0 cm (IQR 2.2-3.9). Extracranial metastases were present in 310 (35.8%) patients, and 365 (42.1%) received stereotactic radiosurgery (SRS) to the resection cavity. No residual on MRI was present in 345 patients (39.9%). In multivariable analysis, STR correlated with decreased IC-PFS (hazard ratio [HR] 1.32, 95% CI 1.13-1.55, P < .001) and OS (HR 1.28, 95% CI 1.08-1.53, P = .005) and a higher occurrence of LMD (odds ratio 1.74, 95% CI 1.10-2.76, P = .02). STR + SRS also correlated with decreased IC-PFS compared with gross total resection (HR 1.35, 95% CI 1.02-1.66, P = .04). In subgroup analysis, the strongest association between EOR and outcomes was observed in large (>3 cm) BMs, supratentorial BMs, and patients without extracranial disease. The use of cavity SRS or the number of BMs had little impact on this association. CONCLUSION: Residual tumor on postoperative MRI after BM resection correlated with worse IC-PFS and OS after adjusting for confounding variables. An increased prevalence of LMD may be a possible mechanism through which these patients experience worse outcomes.