Display omitted •α,β-unsaturated carbonyl moieties in humulanolides play an important role for anti-proliferative activity.•Humulanolide analog E is a novel HSP90 inhibitor.•Inhibition of HSP90 function would be responsible for anti-proliferative activity of humulanolide analog E. Humulanolides are natural products isolated from Asteriscus, and the isolation and total synthesis of many types of humulanolides have been reported. In this study, we evaluated anti-proliferative activity of twelve humulanolides against various human cancer cell lines and found that humulanolide analog E, which was newly designed and synthesized, exhibited the highest anti-proliferative activity. Structure-activity relationship analysis revealed that α,β-unsaturated carbonyl moieties in humulanolides play an important role for anti-proliferative activity. To identify molecular targets of humulanolide analog E, we investigated various cell-based and in vitro assays. Treatment with humulanolide analog E against human fibrosarcoma HT1080 cells increased the expression level of HSP70 protein and decreased the levels of AKT and CDK4, which are HSP90 client proteins. Moreover, humulanolide analog E inhibited refolding of denatured luciferase protein via suppression of HSP90 activity in vitro. These results suggest that humulanolide analog E possesses the anti-proliferative activity against human cancer cells by inhibiting HSP90 functions.