Endometrial cancer is the most common malignant tumors of gynecologic neoplasms in Western society. In recent years, the incidence of endometrial cancer has increased, and it has become the third most common female gynecological cancer (after ovarian and cervical cancer) in Taiwan. Adlay (Coix lachryma-jobi L. var. Ma-yuen Stapf.) has been demonstrated to have bioactive polyphenols, flavonoids, phytosterols, and essential nutrients for health benefits, including anticancer effects in humans. However, little is known about the effect of adlay seeds on endometrial cancer. Our study aimed to investigate the potential growth inhibitory effects of several adlay seed fractions, including ethyl acetate (ATE-EA) and its bioactive constituents, separately on endometrial cancer cells—HEC-1A (phosphatase and tensin homolog-positive) and RL95-2 (phosphatase and tensin homolog-negative)—and identify related active ingredients. In addition, the potential active fractions and the phytochemical compounds were elucidated. The results demonstrate superior activity of ATE-EA with significant in vitro cell proliferation inhibitory capacity, particularly its C.D.E.F-subfraction. Moreover, HPLC- and GC/FID-based quantification of ATE-EA subfractions showed that phenolic compounds (caffeic acid, protocatechuic acid, and p-hydroxybenzaldehyde), flavonoids, steroids, and fatty acid compounds exert anti-proliferative effects in the cell model. Finally, it was shown that cell growth and cell cycle arrest most significantly occurred in the in G1 or G2/M phase under ATE-EA treatment. Collectively, our results demonstrate an antiproliferative effect of ATE-EA on endometrial cancer cells that suggest a positive health outcome for women from consumption of these compounds.