BACKGROUND Circular RNA (circRNA) is a special long-chain non-coding RNA produced during the process of intracellular RNA splicing. Also, circZNF609 is abundant in human tissues, with multiple functions in human diseases, but its role in colorectal cancer remains unknown. This study aimed to evaluate the expression of circZNF609 in tumor tissue and serum samples from patients with colorectal cancer and in colorectal cancer cell lines. MATERIAL AND METHODS The expression of circZNF609 was measured by quantitative polymerase chain reaction (q-PCR) in 45 paired tissue samples from patients with colorectal cancer and 46 serum samples from patients with colorectal cancer and healthy controls, and in the normal human colorectal cell line, FHC, and human colorectal cancer cell lines, HCT116 and HT29. Protein expression of proliferating cell nuclear antigen (PCNA), c-Myc, Bax, Bcl-2, and p53 was determined by Western blot. Cell apoptosis was measured by flow cytometry. RESULTS CircZNF609 was significantly down-regulated in colorectal cancer tissues compared with adjacent normal tissues and in the serum of patients with colorectal cancer compared with healthy controls, verified by receiver operating characteristic (ROC) curve analysis. There was low expression of circZNF609 in HCT116 cells, and overexpression inhibited cell proliferation but had no effect on PCNA and c-Myc protein expression. Expression of circZNF609 induced apoptosis and upregulated expression of the pro-apoptotic protein, Bax, down-regulated the expression of the anti-apoptotic protein, Bcl-2, and upregulated p53. CONCLUSIONS Expression of circZNF609 was down-regulated in colorectal cancer tissue and promoted apoptosis in colorectal cancer cells in vitro by upregulating p53.