The study aims to investigate second primary malignancy (SPM) development and frailty in Turkish geriatric patients with multiple myeloma (MM) and to assess the relationship between overall survival (OS) and various characteristics including SPM and frailty. Seventy-two patients diagnosed with and treated for MM were enrolled in the study. Frailty was determined by the IMWG Frailty Score. Fifty-three participants (73.6%) were found to have clinically-relevant frailty. Seven patients (9.7%) had SPM. Median follow-up was 36.5 (22–48.5) months, and 17 patients died during the follow-up period. Overall (OS) was 49.40 (45.01–53.80) months. Shorter OS was found in patients with SPM (35.29 (19.66–50.91) months) compared to those without (51.05 (46.7–55.4) months) (Kaplan–Meier; p = 0.018). The multivariate cox proportional hazards model revealed that patients with SPM had 4.420-fold higher risk of death than those without (HR: 4.420, 95% CI: 1.371–14.246, p = 0.013). Higher ALT levels were also independently associated with mortality (p = 0.038). The prevalence of SPM and frailty was high in elderly patients with MM in our study. The development of SPM independently reduces survival in MM; however, frailty was not found to be independently associated with survival. Our results suggest the importance of individualized approaches in the management of patients with MM, particularly with regard to SPM development.