Population-based studies on the cause of readmission within 90 days after surgery or radiotherapy for metastatic spine tumors are scarce. We aimed to investigate the risk factors for readmission within 90 days after initial surgical or radiation treatment for metastatic spine tumors. Patients who were diagnosed with metastatic spine tumors between 2012 and 2019 and who underwent spinal magnetic resonance imaging within 1 year were classified according to treatment (surgical or radiotherapy), and the causes for the 90-day readmission and patient characteristics were compared. Overall, 15,815 patients (surgical group, 13,974 patients; radiotherapy group, 1,841 patients) were evaluated. Radiotherapy was preferred in younger and male patients with high Charlson comorbidity index (CCI). Meanwhile, surgery was mainly performed in patients with lumbar metastasis. Radiotherapy, age 30–69 years, male sex, and CCI > 1 were risk factors for 90-day readmission. The main cause of 90-day readmissions included tumor recurrence, chemotherapy, radiotherapy, and treatment of other organ metastasis with radiotherapy. Patients with lung, breast, liver, and colorectal origin and treated with radiotherapy had a high 90-day readmission rate. Radiotherapy, age 30–69 years, male sex, and CCI > 1 increase the risk for 90-day readmission in patients with metastatic spine tumors.