<p>Circular RNAs (circRNAs) have emerged as critical regulators in cancer progression. This study aimed to investigate the role of circ_0003266 in bladder cancer (BCa) and elucidate its underlying molecular mechanisms. The expression levels of circ_0003266, miR-503-5p, and SMAD7 in BCa tissues and cell lines were determined using bioinformatics analysis, clinical data, and quantitative real-time PCR (qPCR). Cellular phenotypes were evaluated through proliferation, migration, and apoptosis assays. RNA pull-down and luciferase reporter assays were conducted to validate the molecular interactions among circ_0003266, miR-503-5p, and SMAD7. The involvement of the TGF-β signaling pathway was assessed by Western blotting and co-immunoprecipitation (Co-IP) analysis. Circ_0003266 expression was significantly downregulated in BCa tissues and cell lines, and its low expression was clinically correlated with larger tumor size and advanced disease stage, including poor differentiation and higher TNM classification. Upregulation of circ_0003266 inhibited BCa cell proliferation and migration while promoting apoptosis. Mechanistic analyses revealed that miR-503-5p is a direct target of circ_0003266, and that circ_0003266-mediated regulation of miR-503-5p subsequently modulates SMAD7 expression. SMAD7, a well-established inhibitor of the TGF-β pathway, was shown to mediate the tumor-suppressive effects of circ_0003266. In vivo experiments confirmed that overexpression of circ_0003266 attenuated tumor growth and suppressed TGF-β signaling activity. Our findings demonstrate that circ_0003266 exerts tumor-suppressive functions in BCa by modulating the miR-503-5p/SMAD7/TGF-β axis, highlighting its potential as a novel therapeutic target for bladder cancer intervention.</p>