Gymnopilus junonius (Fr.) P. D. Orton (Cortinariaceae) is a hallucinogenic mushroom, a well-known poisonous mushroom that is widely known as "big laughter mushroom" because it causes excessive laughter in those who consume it. Chemical investigation of G. junonius fruiting bodies was performed, resulting in the isolation and structural identification of three sesquiterpenes (1-3), including a new trichothecene sesquiterpene (2) and a new tremulane sesquiterpene (3). Compound 1 was identified from G. junonius for the first time. The chemical structures of the new compounds were established by detailed analysis of 1D and 2D (H-H correlated spectroscopy [COSY], heteronuclear single quantum coherence [HSQC], and heteronuclear multiple-bond coherence [HMBC]) nuclear magnetic resonance (NMR) spectra, and high-resolution mass spectrometry (HRMS). In particular, the absolute configurations of compounds 2 and 3 were unambiguously determined by quantum chemical electronic circular dichroism (ECD) calculations. The isolated compounds (1-3) were evaluated for their cytotoxic effects on human lung and prostate cancer cell lines where trichothecene sesquiterpenes (1 and 2) showed remarkable cytotoxicity similar to that of the control drug, i.e., doxorubicin. Our findings provide experimental evidence suggesting the potential anti-cancer effects of trichothecene sesquiterpenes from a poisonous mushroom.