Endoplasmic reticulum (ER) stress, together with unfolded protein response (UPR), can remove unfolded proteins and promote survival. However, severe and prolonged ER stress leads to cell death, tissue injury, and many serious diseases. Therefore, it is essential to identify drugs that can attenuate ER stress for ER-related disease treatment. A great deal of research shows that selenoprotein S (SelS) is a sensitive and ideal marker of ER stress. Here, we used a firefly luciferase reporter driven by SelS gene promoter to screen natural compounds that can attenuate ER stress. Then we identified compound 20(S)-25-methoxyl-dammarane-3β,12β,20-triol (25-OCH3-PPD) could inhibit the promoter activity of SelS, further results showed that 25-OCH3-PPD effectively inhibited tunicamycin (TM) induced up-regulation of SelS expression in both mRNA and protein levels. Moreover, 25-OCH3-PPD significantly inhibited glucose-regulated protein 78 (GRP78; the major ER stress marker) expression in TM-induced ER stress in HepG2 and HEK293T cells, suggesting that 25-OCH3-PPD could attenuate ER stress in these cells. Mechanism studies showed that 25-OCH3-PPD significantly activated ERK/MAPK signaling pathway, and the inhibition of ERK/MAPK by U0126 dramatically abolished the inhibitory effect of 25-OCH3-PPD on ER stress, suggesting that 25-OCH3-PPD attenuated ER stress at least partially through activation of ERK/MAPK signaling pathway. Taken together, our studies indicate that 25-OCH3-PPD is a novel small molecular compound reducing ER stress, and a potential drug for treating diseases associated with ER stress.