Aim: The Ewing sarcoma family of tumors (ESFT) is a group of malignant small round cell neoplasms of bones and soft tissues closely histogenetically related. Methylthioadenosine phosphorylase (MTAP) deficiency has been recently associated with increased tumor aggressiveness and poor outcomes in different types of neoplasms. However, the expression of this biomarker and its biological role in ESFT remain largely unknown. Methods: Immunohistochemical expression of MTAP was accessed in 112 patients with ESFT in a tissue microarray platform and associated with clinicopathological parameters and overall survival (OS). Results: Loss of MTAP expression was significantly associated with lower OS in both univariate and multivariate analyses. Conclusion: Loss of MTAP expression is an independent negative prognostic biomarker in ESFT.