The aim of this study was to evaluate the relationship between prognostic nutritional index (PNI) and systemic immune‐inflammation index (SII) and clinical features and prognosis of osteosarcoma patients. We retrospectively investigated 126 patients with surgery for osteosarcoma between 2012 and 2018 at our hospital. The preoperative PNI was calculated as albumin level (g/L) + 5 × total lymphocyte count (10
9/L). The SII was defined as platelet × neutrophil/lymphocyte counts. The optimal cut‐off values for PNI and SII were evaluated with receiver operating curve analysis. Clinical features and PNI and SII were tested with the
χ
2 test. The effects of PNI and SII on overall survival (OS) was investigated by Kaplan–Meier method and Cox proportional hazards model. A low preoperative PNI was remarkably correlated with tumor size, Enneking stage, pathological fracture, local recurrence, metastasis, and neoadjuvant chemotherapy (
p < 0.05). Whereas, a high SII was significantly associated with tumor size, histological type, Enneking stage, and neoadjuvant chemotherapy (
p < 0.05). There was a significant negative relationship between the PNI and SII (
r = 0.384;
p < 0.001). For univariate analyses, the results revealed that tumor size, local recurrence, metastasis, PNI, and SII were predictors of OS (
p < 0.05). In multivariate analyses, local recurrence (
p = 0.010), metastasis (
p < 0.001), PNI (
p < 0.001), and SII (
p = 0.029) as independent prognostic factors were significantly correlated with OS. This study suggested that PNI and SII could be important prognostic parameters for patients with osteosarcoma.