Malnutrition is a severe but modifiable risk factor for cancers. However, the relationship between malnutrition and the survival of patients with brain metastases has not been fully revealed. We aimed to evaluate the prevalence of malnutrition and assess its prognostic value on patients with brain metastases.We retrospectively recruited 2,633 patients with brain metastases between January 2014 and September 2020. Three malnutrition scores were used to evaluate patients' malnutrition status at their first admission, including controlling nutritional status, the nutritional risk index, and the prognostic nutritional index. The association between malnutrition and overall survival (OS) was estimated.The three malnutrition scores were associated with each other and with body mass index (BMI). Malnutrition assessed by any of the three scores was significantly associated with poor OS. All three malnutrition scores were better indicators than BMI, and adding malnutrition scores to the Graded Prognostic Assessment (GPA) scoring system could significantly improve the accuracy of prognosis prediction.Malnutrition monitoring using any of the three malnutrition scores on patients' first admission could be a better survival indicator for patients with brain metastases compared with BMI alone.Malnutrition is a more significant indicator of survival stratification compared with BMI. Adding malnutrition to the GPA score system achieves better survival prediction.