Heterogeneous nuclear ribonucleic protein K (hnRNPK) regulates the expression of various genes, but has contradictory roles as a tumor promoter and a tumor suppressor. We recently reported that the expression of hnRNPK is negatively associated with malignant behavior of breast cancer where it was induced by estrogen, and bound to estrogen receptor α (ERα) in the nucleus of breast cancer cells. However, the significance of hnRNPK in endometrial cancer, also an estrogen-dependent cancer, remains unclear. In this study, we first examined the localization of hnRNPK and ERα in normal endometrium and endometrial cancer. hnRNPK and ERα immunoreactivity was detected in the nuclei of endometrial glandular and carcinoma cells. In normal endometria, hnRNPK labeling index/immuno-intensity was significantly higher in the proliferative phase than in the secretory phase. In endometrial cancer tissues, hnRNPK labeling index/immuno-intensity was significantly higher in the adjacent non-malignant glandular cells compared to that in carcinoma cells. Immunohistochemistry results for ERα were identical to that of hnRNPK both in normal endometrium and endometrial cancer. In normal and cancerous tissues, the median value of the hnRNPK labeling index was significantly higher in the ERα-high group. Intratumoral estrogen, but not androgen, measured using liquid chromatography-tandem mass spectrometry, was significantly positively correlated with the hnRNPK labeling index in endometrial cancer tissues. Database analysis revealed that the hnRNPK high expression group had a significantly better prognosis for both overall and disease-free survival. These results suggest that hnRNPK interacts with ERα to regulate endometrial changes during the menstrual cycle and suppress the malignant behavior of endometrial cancer.