The diagnosis and screening indicators for gastric cancer (GC) are not satisfactory, resulting in a large number of GC patients being missed and missing the best treatment time. Due to the special structure of circular RNAs (circRNAs), they have a more accurate and powerful ability to detect tumor occurrence. In addition, circHAS2 has been found to promote the proliferation, migration, and invasion of GC cells. Therefore, this study explored the potential of circHAS2 as a biomarker for GC. The expression level of circHAS2 in the specimens was detected by real‐time fluorescent quantitative PCR. The molecular characteristics of circHAS2 were verified by agarose gel electrophoresis and Sanger sequencing. The feasibility of the circHAS2 detection method was verified by room temperature placement and repeated freezing and thawing. The diagnostic effect of circHAS2 on GC was evaluated by receiver‐operating curve analysis. The correlation between circHAS2 expression level and clinical pathological parameters was analyzed using the χ2‐test. Kaplan–Meier survival curve analysis was used to analyze the survival situation of the circHAS2 high‐ and low‐expression group. Univariate and multivariate Cox regression analysis was used to evaluate the influencing factors of prognosis in GC patients. CircHAS2 in cells can be secreted into the blood, and its expression level is significantly upregulated in the serum of patients with GC. The expression level of circHAS2 is correlated with the tissue differentiation, tumor node metastasis staging, classification, and lymph node metastasis of GC patients. CircHAS2 can effectively identify GC and even early GC. In addition, the expression levels of circHAS2 in postoperative GC patients significantly decreased and returned to normal after the second stage of chemotherapy. Finally, the circHAS2 low‐expression group had better survival. The upregulated expression of circHAS2 in the serum of GC patients can effectively identify GC and early GC and can be used for effective monitoring of the prognosis of GC patients. In summary, circHAS2 can be used as an effective diagnostic and prognostic marker for GC.