Acute kidney injury (AKI), a kind of syndrome with sudden degradation of renal function, is characterized by loss of urine creatinine. 1 With 7.0% incidence in China, AKI becomes a representative public health matter that ranges from newborns to senior citizens. 2 AKI is divided into pre-kidney, intrinsic and post-kidney, of whose pathophysiology risk factors include inflammatory diseases, 3 traumatism 4 and drug overdose. 5 In addition, ischaemia/reperfusion (I/R) during kidney transplantation also is extremely likely to elicit AKI. 6,7 I/R injury, which results from the inadequate supply of oxygen and nutrients to kidney Abstract Circular RNA YAP1 (circYAP1) was reported to participate in progression of gastric cancer. However, the role of circYAP1 in acute kidney injury (AKI) remains obscure.We attempted to examine the effects of circYAP1 on ischaemia/reperfusion-stimulated renal injury. AKI model was established by treating HK-2 cells in ischaemia/ reperfusion (I/R) environment. CircYAP1 expression in blood of AKI patients and I/Rtreated HK-2 cells was evaluated via RT-qPCR. CCK-8, flow cytometry, ELISA and ROS assay were executed to test the impact of circYAP1 on cell viability, apoptosis, inflammatory cytokines and ROS generation. Bioinformatic analysis was executed to explore miRNA targets. The relativity between circYAP1 and miR-21-5p was verified by RT-qPCR and luciferase assay. The functions of miR-21-5p in I/R-triggered injury were reassessed. PI3K/AKT/mTOR pathway was detected by Western blot. Down-regulated circYAP1 was observed in AKI blood samples and I/R-treated HK-2 cells. CircYAP1 overexpression expedited cell growth and weakened secretion of inflammatory factors and ROS generation in I/R-disposed cells. Besides, we found circYAP1 could sponge to miR-21-5p. Interestingly, miR-21-5p overexpression overturned the repressive effects of circYAP1 on cell injury. Moreover, PI3K/AKT/mTOR pathway was activated by circYAP1 via inhibiting miR-21-5p. We demonstrated that circYAP1 activated PI3K/AKT/mTOR pathway and secured HK-2 cells from I/R injury via sponging miR-21-5p. K E Y W O R D S acute kidney injury, circular RNA YAP1, ischaemia/reperfusion, microRNA-21-5p, PI3K/AKT/ mTOR pathway