Circular RNA (circ)_0000326 has been reported in bladder cancer and cervical cancer and is concerned to be involved with the development of cancerous cells. Whereas, there have been no reports concentrating on the influences of circ_0000326 in breast cancer (BC). Therefore, the latent modulatory mechanisms of circ_0000326 in BC are researched. circ_0000326 expression in BC tissues and correlative cells was evaluated via RT-qPCR, and the relevance between circ_0000326 expression and overall survival and the clinicopathological feature was also investigated. After a series of transfection, the effects of circ_0000326, microRNA-9-3p (miR-9-3p), and Yes-associated protein 1 (YAP1) in BC cell growth, invasion, and stemness were studied by CCK-8, flow cytometry, Transwell, and sphere-forming assays. The binding sites and correlation of circ_0000326, miR-9-3p, and YAP1 were certified via starBase website, luciferase reporter assay, and Pearson's χ2 test. The in vivo experiment was evaluated by establishing a subcutaneous tumorigenesis model. High-expressed circ_0000326 in BC tissues and cells was discovered, which was connected with an undesirable prognosis. Silencing of circ_0000326 visibly inhibited MCF-7 and BT549 cell growth, invasion, stemness, meanwhile declining the protein levels of SRY-related high-mobility group box gene 2 (SOX2) and octamer binding transcription factor 4 (OCT4). miR-9-3p was a sponger of circ_0000326, which was negatively regulated by circ_0000326. Moreover, YAP1 was confirmed as a target gene of miR-9-3p. circ_0000326 affected BC cell behaviors via mediating miR-9-3p and YAP1. Furthermore, circ_0000326 silencing prohibited tumor growth of BC in vivo. The research uncovered that circ_0000326 facilitated BC development via mediating the miR-9-3p/YAP1 axis.