This study aimed to investigate the association between marital status and the prognosis of patients with metastatic breast cancer (MBC). Data of patients with MBC were obtained from the Surveillance, Epidemiology, and End Results (SEER) database. Patients were classified into married and unmarried groups. Kaplan–Meier analysis with the log-rank test was conducted to compare breast cancer-specific survival (BCSS) and overall survival (OS) between the groups. Univariable and multivariable Cox proportional models were used to determine whether marital status was independently associated with OS, and the Fine–Gray subdistribution hazard method was performed to determine whether marital status was independently associated with BCSS. In total, 16,513 patients with MBC were identified, including 8949 married (54.19%) and 7564 unmarried (45.81%) patients. The married patients were significantly younger [median age (interquartile range), 59.0 (50.0–68.0) vs. 63.0 (53.0–75.0); p < 0.001] and received more aggressive treatments, such as chemotherapy (p < 0.001) and surgery (p < 0.001), than the unmarried patients. Moreover, married patients had higher 5-year BCSS (42.64% vs. 33.17%, p < 0.0001) and OS (32.22% vs. 21.44%, p < 0.0001) rates. Multivariable analysis revealed that marital status was an independent prognostic factor, and married status was associated with a significant reduction in the risk of breast cancer-specific (sub-hazard ratio, 0.845; 95% confidence interval, 0.804–0.888; p < 0.001) and all-cause (hazard ratio, 0.810; 95% confidence interval, 0.777–0.844; p < 0.001) mortality. Unmarried patients had a 15.5% increased risk of breast cancer-specific mortality and a 19.0% increased risk of overall mortality compared with married patients with MBC. BCSS and OS were superior in married populations compared with unmarried populations in most subgroups. Marital status was an independent prognostic indicator for survival in patients with MBC and was associated with significant survival benefits.