Cancer stem cells (CSCs) play a central role in cancer initiation, progression, therapeutic resistance, and recurrence in patients. Here we present Capicua (CIC), a developmental transcriptional repressor, as a suppressor of CSC properties in breast cancer cells. CIC deficiency critically enhances CSC self-renewal and multiple CSC subpopulations of breast cancer cells without altering their growth rate or invasiveness. Loss of CIC relieves repression of ETV4 and ETV5 expression, consequently promoting self-renewal capability, EpCAM+/CD44+/CD24low/- expression, and ALDH activity. In xenograft models, CIC deficiency significantly increases CSC frequency and drives tumor initiation through derepression of ETV4. Consistent with the experimental data, the CD44high/CD24low CSC-like feature is inversely correlated with CIC levels in breast cancer patients. We also identify SOX2 as a downstream target gene of CIC that partly promotes CSC properties. Taken together, our study demonstrates that CIC suppresses breast cancer formation via restricting cancer stemness and proposes CIC as a potential regulator of stem cell maintenance.