The involvement of New York esophageal squamous cell carcinoma-1 (NY-ESO-1) and melanoma-associated antigen A4 (MAGE-A4) in soft-tissue sarcoma pathogenesis has recently been reported; however, their involvement in desmoid tumors (DTs) remains unknown. This study aimed to determine the involvement of NY-ESO-1 and MAGE-A4 in DTs. Immunostaining for β-catenin, NY-ESO-1, and MAGE-A4 was performed on DT biopsy specimens harvested at our institution. The positivity rate for each immune component was calculated. In addition, the correlations between the positivity rates for the immune molecules were investigated. The correlation between the positivity rate and age or longest diameter of each immune molecule was also investigated. β-catenin showed staining mainly in the tumor cell nuclei of DTs. Both NY-ESO-1 and MAGE-A4 showed staining in the nucleus, cytoplasm, and infiltrating lymphocytes of DT cells. The mean positive cell rates for β-catenin, NY-ESO-1, and MAGE-A4 were 43.9 ± 21.7, 30 ± 21.6, and 68.9 ± 20.8, respectively. A strong negative correlation was observed between β-catenin and MAGE-A4 positivity rates (r= −0.64). The positivity rates for NY-ESO-1 and MAGE-A4 showed a moderate positive correlation (r= −0.42). A very strong negative correlation was observed between age and the NY-ESO-1 positivity rate (r= −0.72). A weak negative correlation was observed between age and the MAGE-A4 positivity rate (r= −0.28). A medium negative correlation was observed between the longest tumor diameter and NY-ESO-1 positivity (r= −0.37). NY-ESO-1 and MAGE-A4 may be involved in the DT microenvironment. Thus, NY-ESO-1 and MAGE-A4 may be useful in the diagnosis of DT.