BackgroundGlobally, gastric cancer (GC) is one of the world’s most widespread malignancies, with persistent high mortality and morbidity rates. Increasing evidence now suggests that microRNAs (miRNAs) participate in many biological processes, with miR-455-3p having key roles in the progression of diverse cancers. Nevertheless, miR-455-3p function and expression in GC remain unclear.MethodsWe explored miR-455-3p expression in GC using quantitative polymerase chain reaction (qPCR). To further examine the effect of miR-455-3p in GC, after transfecting miR-455-3p mimics or inhibitors into GC cells, 5-ethynyl-2’-deoxyuridine (EdU) incorporation and colony formation assays were performed to examine cell proliferation. Flow cytometry was used to detect apoptosis, and expression levels of Bax, Bcl-2, Snail, N-cadherin, E-cadherin, and Caspase-3 were assessed by western blotting (WB). Using online databases and luciferase assays, we identified armadillo repeat-containing protein 8 (ARMC8) as a promising target of miR-455-3p. A mouse tumor model was established to investigate actions of miR-455-3p in vivo. Expression levels of C-myc, cyclinD1, and β-catenin were examined using WB and immunofluorescence.ResultsMiR-455-3p expression was attenuated in GC tissue and cell lines. MiR-455-3p overexpression inhibited GC cell proliferation, epithelial-mesenchymal transition (EMT), as well as facilitated apoptosis, while suppression of miR-455-3p had the opposite effects. From luciferase assays, we confirmed that ARMC8 was a novel and direct downstream target gene of miR-455-3p, and that the tumor suppressive role of miR-455-3p was in part reversed due to ARMC8 overexpression. Moreover, miR-455-3p inhibited GC growth in vivo via ARMC8. We also observed that miR-455-3p repressed canonical Wnt pathway activation by binding to ARMC8.ConclusionsMiR-455-3p exerted tumor inhibitory effects in GC by targeting ARMC8. Therefore, intervening in the miR-455-3p/ARMC8/Wnt/βcatenin axis could be a promising novel treatment strategy for GC.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12920-023-01583-y.