Abstract: Cancer represents a significant disease that profoundly impacts human health and longevity. Projections indicate a 47% increase in the global cancer burden by 2040 compared to 2020, accompanied by a further rise in the associated economic burden. Consequently, there is an urgent need to discover and develop new alternative drugs to mitigate the global impact of cancer. Natural products (NPs) play a crucial role in the identification and development of anticancer therapeutics. This study identified ustusolate E (UE) and its analog 11α-hydroxy-ustusolate E (HUE) from strain Aspergillus calidoustus TJ403-EL05, and examined their antitumor activities and mechanisms of action. The findings demonstrate that both compounds significantly inhibited the proliferation and colony formation of AGS (human gastric cancer cells) and 786-O (human renal clear cell carcinoma cells), induced irreversible DNA damage, blocked the cell cycle at the G₂/M phase, and further induced apoptosis in tumor cells. To the best of the authors’ knowledge, this is the first report on the anticancer effects of UE and HUE and their underlying mechanisms. The present study suggests that HUE and UE could serve as lead compounds for the development of novel anticancer drugs.