Endometrial cancer (EC) is one of the most common gynaecological malignant tumours with a high incidence, leading to urgent demands for exploring novel carcinogenic mechanisms and developing rational therapeutic strategies. The rac family of small GTPase 3 (RAC3) functions as an oncogene in various human malignant tumours and plays an important role in tumour development. However, the critical roles of RAC3 in the progression of EC need further investigation. Based on TCGA, single-cell RNA-Seq, CCLE and clinical specimens, we revealed that the RAC3 was specifically distributed in EC tumour cells compared to normal tissues and functioned as an independent diagnostic marker with a high area under curve (AUC) score. Meanwhile, the RAC3 expression in EC tissues was also correlated with a poor prognosis. In detail, the high levels of RAC3 in EC tissues were reversely associated with CD8+ T cell infiltration and orchestrated an immunosuppressive microenvironment. Furthermore, RAC3 accelerated tumour cell proliferation and inhibited its apoptosis, without impacting cell cycle stages. Importantly, silencing RAC3 improved the sensitivity of EC cells to chemotherapeutic drugs. In this paper, we revealed that RAC3 was predominantly expressed in EC and significantly correlated with the progression of EC via inducing immunosuppression and regulating tumour cell viability, providing a novel diagnostic biomarker and a promising strategy for sensitizing chemotherapy to EC.© 2023 The Authors. Journal of Cellular and Molecular Medicine published by Foundation for Cellular and Molecular Medicine and John Wiley & Sons Ltd.