Purpose: Multiple myeloma (MM), a clonal plasma cell malignancy, composes around 10% of hematologic malignancies. Though recent advances in treatment have dramatically improved MM survival, some aggressive courses of disease and dismal outcomes still exist. Low body weight, undernutrition, and cachexia are noted at MM diagnosis. We aim to evaluate the impact of low body mass index (BMI) and undernutrition in MM patients. Methods: We retrospectively analyzed MM patients at Taipei Veterans General Hospital in Taiwan between January 1, 2006 and October 31, 2018. Being underweight is defined as having a BMI of under 18.5 kg/m2. The patient’s baseline characteristics, including BMI, serum albumin level, and comorbidities, etc., were recorded. The primary endpoint of the study was all-cause mortality. A Cox regression model was used to estimate the risk factors of mortality. Results: A total of 378 newly diagnosed MM patients were enrolled in this study. The median age of the patients was 69. Thirty patients (7.9%) were underweight at diagnosis. The median overall survival was 1.3 years (95% CI 0.3–5.7) and 5.0 years (95% CI 3.1–5.9) for patients with low BMI and for patients with normal or higher BMI, respectively. In the multivariate analysis, low BMI (95% CI 1.07–4.44), ECOG ≥2 (95% CI 1.02–2.89), hypoalbuminemia (95% CI 1.21–4.01), high LDH (95% CI 1.22–3.49), and light chain ratio > 100 (95% CI 1.06–2.77) were independent risk factors of mortality. Conclusion: MM patients who were underweight, with hypoalbuminemia, poor performance status, higher LDH, and light chain ratio > 100 were associated with poor overall survival.