ABSTRACT Purposes: The aim of this study was to determine the diagnostic significance of fibronectin type III domain containing protein 5 (FNDC5)/Irisin levels in the sera of patients with renal cell cancer. Materials and Methods: In the study, 48 individuals were evaluated. The patient group included 23 subjects diagnosed with renal tumor, and the control group of 25 healthy individuals. Patients diagnosed with renal tumor received surgical treatment consisting of radical or partial nephrectomy. Blood specimens were collected and serum FNDC5/Irisin and carcinoembryonic antigen (CEA) levels were determined using enzyme-linked immunosorbent assay (ELISA). Results: FNDC5/irisin and CEA levels in renal cancer patients were significantly higher compared with the control group (p=0.0001, p=0.009, respectively). Also, FNDC5 levels was more sensitive and specific than CEA levels. The best cut-off points for FNDC5/irisin were >105pg/mL and CEA were >2.67ng/mL for renal cancer. Conclusions: FNDC5/Irisin may be used as a diagnostic biomarker for renal cancer.