<p>This study investigated the expression, methylation patterns, and clinicopathological implications of ARHGAP40 in renal cell carcinoma (RCC), the most common urinary malignancy. A total of 60 clear cell renal cell carcinomas (ccRCC), 30 papillary renal cell carcinomas (pRCC), 30 chromophobe renal cell carcinomas (chRCC), and 13 other RCC subtypes were enrolled. ARHGAP40 expression was analyzed in both RCC tissues and matched paracancerous normal tissues using immunohistochemistry (IHC). The methylation status of the ARHGAP40 promoter region was assessed in both normal and tumor samples by bisulfite sequencing PCR (BSP). Circulating tumor DNA (ctDNA) extracted from peripheral blood samples of RCC patients (20), patients with benign renal tumors (1), and healthy controls (14) was quantitatively analyzed for methylation using quantitative methylation-specific PCR (qMSP). ARHGAP40 expression was significantly downregulated in RCC compared to matched normal tissues (<i>P</i> &lt; 0.001). This reduced expression correlated with tumor necrosis (<i>P</i> = 0.009) but showed no significant association with age, gender, tumor location, tumor diameter, TNM stage, or vascular invasion. In the ccRCC subtype, ARHGAP40 expression exhibited a progressive decrease with larger tumor diameter (<i>P</i> = 0.045), advancing histological grade (<i>P</i> = 0.032), and tumor necrosis (<i>P</i> = 0.011). The methylation status of ARHGAP40 was consistent with its expression level in both tumor and adjacent normal tissues. Methylated ARHGAP40 DNA was detectable only in RCC patient ctDNA samples. ARHGAP40 is epigenetically silenced in RCC through methylation-mediated downregulation, which correlates with tumor necrosis and grade. The detection of methylated ARHGAP40 in ctDNA holds promise as a potential biomarker for early RCC diagnosis.</p>