ABSTRACT Background Merkel cell carcinoma (MCC) is an aggressive skin cancer, related to the Merkel Cell Polyomavirus (MCPyV) in 80% of cases. Immune checkpoint inhibitors provide sustained benefit in about half of MCC patients with advanced disease. Glypican-3 (GPC3) is an oncofetal tumor antigen that is an attractive target for chimeric antigen receptor T cell therapy due to its highly restricted expression on normal tissue and high prevalence in several solid tumors. GPC3 was previously found to be expressed in MCC but its association with tumor characteristics or prognosis has not been reported. Objectives To investigate the expression of GPC3 in MCC by immunohistochemistry (IHC) and its association with tumor characteristics, MCPyV status, and patient outcome. Methods The GC33 antibody clone was validated for GPC3 IHC staining of tumor specimens in comparison to an established GPC3 IHC antibody. A tissue microarray of tumors collected from an ongoing cohort of MCC patients was stained for GPC3 by IHC using GC33 antibody. Association of GPC3 positivity with baseline characteristics, MCPyV status (quantitative PCR) and outcome (death from MCC, recurrence) were assessed by Fisher’s exact tests and Cox regression analysis. Results Among 62 tumors from 59 patients, 42 samples (67.7%) were GPC3-positive. GPC3 expression was more frequently observed in females (p=0.048) and MCPyV-negative tumors (p=0.021). In the multivariate analysis, GPC3 expression was associated with increased death from disease (CSS) (hazard ratio [HR] 4.05, 95% CI 1.06-15.43), together with advanced age (HR 4.85, 95% CI 1.39-16.9) and male gender (HR 4.64, 95% CI 1.31-16.41). Conclusions GPC3 expression is frequently expressed in MCC tumors, especially MCPyV-negative cases, and is associated with increased risk of death. The high prevalence of surface GPC3 makes it a putative drug target.