In clinical practice, pirarubicin (THP) is a widely used triple-negative breast cancer (TNBC) agent. It has been found that circular RNAs (circRNAs) are involved in cancer treatment and progression. However, the biological function of circRNAs in TNBC and the relationship between THP and circRNAs remain poorly studied. circSTIL (hsa_circ_0000069) was screened and validated by bioinformatics analysis, demonstrating that it was highly expressed in TNBC cell lines and plasma exosomes, and correlated with a poor prognosis of patients. The expression level of circSTIL in patients' plasma exosomes has potential diagnostic value in distinguishing TNBC from non-TNBC. In vitro studies confirmed that overexpression of circSTIL promotes the proliferation, migration, and invasion of MDA-MB-231 cells whereas silicification of circSTIL shows the reverse effect. Also, circSTIL mediates THP inhibiting the malignant phenotype of MDA-MB-231 cells. The above results suggested that circSTIL is a possible biomarker for the diagnosis, treatment, and prognosis of TNBC.