Administration of Dienogest prior to hysteroscopic polypectomy is empirically performed, but the physiological effects of Dienogest on endometrial polyps are unclear. We aimed to investigate the effects of Dienogest on the proliferation and inflammation of endometrial polyps. We conducted a retrospective case study on 40 menstruating women who underwent hysteroscopic polypectomy at our hospital. We collected clinical data, and the polyps were divided by morphological appearance. The specimens obtained were immunostained for Ki67 as a marker of cellar proliferation and CD138 as a marker of plasmacytes, which are a hallmark of chronic endometritis. Dienogest significantly suppressed the proliferation status of EPs because Dienogest treatment prior to the operation significantly reduced the Ki67 index (41.25 ± 16.85 vs 7.18 ± 9.82, p < 0.01). We found that sessile-type polyps showed a significantly lower Ki67 index than the pedunculated type (12.28 ± 11.12 vs 2.09 ± 2.73, p = 0.026). The presence of CD138-positive cells was more pronounced in sessile-type polyps than in pedunculated polyps (p = 0.018). However, Dienogest treatment showed no apparent effect on inflammation status, as detected by CD138-positive cells. We revealed that Dienogest suppressed cellular proliferation, and morphological classification of endometrial polyps could be used to predict the responsiveness to Dienogest. However, Dienogest might not affect cellular inflammation.