The role of postmastectomy radiotherapy (PMRT) in patients with de novo stage IV breast cancer is unclear. This study aimed to evaluate the value of PMRT for metastatic breast cancer who underwent a modified radical mastectomy. Data on de novo stage IV breast cancer patients who received modified radical mastectomy between 2010 and 2015 were retrieved from the Surveillance, Epidemiology, and End Results (SEER) database. Propensity score matching (PSM) analysis based on age, T stage, N stage, breast subtype, and chemotherapy was conducted to balance baseline clinical characteristics. The prognostic roles of PMRT on cancer-specific survival (CSS) and overall survival (OS) were analyzed using the Kaplan-Meier method and Cox proportional hazard models. A total of 1944 patients were enrolled before PSM. After PSM, 1458 patients were included. PMRT improved the prognosis of CSS and OS. Multivariate Cox analysis showed that PMRT was independently prognostic for CSS (HR 0.739, 95% CI. 0.619-0.884, P = 0.001) and OS (HR 0.744, 95%CI 0.628-0.8810, P = 0.001). Further subgroup analyses found that survival superiority was observed in T3-4 or N + subgroup (both P < 0.001 for CSS and OS), and Her2-/HR + breast subtype (HR 0.703, 95%CI 0.558-0.888 for CSS, and HR 0.712, 95%CI 0.573-0.885 for OS), especially in patients with bone metastasis but without brain metastasis. PMRT improved survival in de novo stage IV breast cancer patients in selected T3-4 or N + subgroup and Her2-/HR + breast subtype. However, these findings need to be validated by further studies before being incorporated into clinical practice.