Multiple myeloma (MM) derives from the clonal proliferation of plasma cells, primarily residing in the bone marrow. However, MM cells can disseminate systemically, leading to osseous or soft tissue extramedullary disease (EMM) or plasma cell leukemia (PCL). The presence of EMM or PCL has historically been linked to poor prognosis and aggressive features. In this study, we analyzed 201 patients with EMM treated at our institution between January 1, 2010, and November 30, 2023. Among these patients, 25 had primary PCL, 19 had secondary PCL, 89 were diagnosed with EMM at the time of MM diagnosis, 29 developed EMM after therapy, and 39 had solitary plasmacytoma (SP), with 20 progressing into MM. Patients with EMM at the time of MM diagnosis or SP progressing to MM exhibited a median overall survival (OS) comparable to those with MM alone (7.5 years or not reached). However, the presence of EMM was associated with worse prognosis in specific groups: primary PCL (median OS: 26 months), secondary PCL (median OS: 1.6 months), and secondary EMM (median OS: 16 months). Additional prognostic features included high R-ISS (Revised International Staging System), chromosomal abnormalities (1q+, 17p deletion, and 13q deletion), and elevated lactate dehydrogenase values at presentation. While the site of EMM did not correlate with inferior outcomes, osseous SP increased the risk of progression to overt MM. In conclusion, the presence of EMM confers variable prognosis, emphasizing the need for more effective therapeutic strategies, particularly for patients with PCL or those developing EMM later during treatment.