The aim of this study was to investigate the role of microRNA-21 (miR-21) in the regulation of phosphatase and tensin homolog deleted from chromosome-10 (PTEN) expression and proliferation of endometrioid endometrial cancer (EEC) cells. We performed a qRT-PCR assay with miR-21 and PTEN in 16 paired EEC tumor tissues and adjacent non-tumor endometrium. To investigate the regulation of PTEN by miR-21, we designed gain- and loss-of-function of miR-21 experiments in the KLE cell line by transfection with a synthetic miR-21 mimic and inhibitor. To validate the putative binding site of miR-21 in the 3' untranslated region (3'-UTR) of PTEN messenger RNA (mRNA), a dual-luciferase reporter assay was carried out. To evaluate the potential effect of miR-21 on EEC proliferation, we performed both overexpression experiments, using an miR-21 mimic, and inhibition assays, using an miR-21 inhibitor. miR-21 was overexpressed in EEC and was inversely correlated with PTEN protein expression (P<0.001). miR-21 regulated PTEN protein expression and cell proliferation in the KLE cell line and the direct binding of miR-21 to the PTEN 3'-UTR was confirmed using a dual-luciferase reporter assay. The upregulation of miR-21 led to a significant decrease in the PTEN protein expression level (P=0.007). The downregulation of miR-21 led to a significant increase in PTEN protein (P=0.002). The expression of luciferase in the wt-PTEN-3'-UTR-pGL3 group was downregulated in the presence of the miR-21 mimic (P=0.001). miR-21 was overexpressed in EEC. In conclusion, we demonstrated that the expression of PTEN protein, but not mRNA, was negatively directly regulated by miR-21 in the KLE cell line. The overexpression of miR-21 modulated EEC cell proliferation through the downregulation of PTEN.