Objective To explore the expression and function of circular RNA (circRNA) circNEIL3 in breast cancer (BC) and its influence on the expression levels of cell cycle-related proteins. Methods Clinical and pathological data from 50 patients with pathologically confirmed BC, along with samples of their BC tissues and para-cancer tissues were collected. circRNA sequencing was used to analyze the differentially expressed circRNAs in 3 pairs of BC tissues and para-cancer tissues. The characteristic of differentially expressed circRNA circNEIL3 was verified by ribonuclease (RNase) digestion and cytoplasmic/nuclear fractionation experiments. The expression of circNEIL3 in BC tissues and cells was detected by quantitative polymerase chain reaction. The relationship between circNEIL3 expression and the patients’ clinicopathological data was also analyzed. The effect of circNEIL3 on proliferation of BT-549 and MCF-7 cells was identified by cell counting kit 8 and colony-formation assays. The changes of cell apoptosis and cell cycle were detected by flow cytometry. The expression levels of cyclin D1 (CCND1), cyclin-dependent kinase (CDK) 4, cyclin E1 (CCNE1) and CDK2 were measured by Western blotting. Results circNEIL3 differentially expressed in BC tissues and adjacent tissues was not easily degraded by RNase R and mainly located in the cytoplasm of BC cells. Compared with normal breast tissues and cells, the expression levels of circNEIL3 in BC tissues and cells were significantly up-regulated (all P＜0.05). circNEIL3 was highly expressed in BC patients with tumor maximum diameter＞2 cm and at high TNM stage (stage Ⅲ) (both P＜0.05). Overexpression of circNEIL3 enhanced proliferation of BC cells and up-regulated the levels of CCND1, CDK4, CCNE1 and CDK2 (all P＜0.05). Knockdown of circNEIL3 induced cell apoptosis (P＜0.01), resulting in cell cycle arrest at G1 phase (P＜0.01). Conclusion High expression of circNEIL3 in BC tissues and cells can promote the proliferation of BC cells by up-regulating the expression levels of cycle-related proteins.