Omphalia lapidescens Schroet. is a medicinal macrofungus in China that has shown good antitumor activity in recent research. To explore the potential cytotoxic compounds from O. lapidescens , the ethyl acetate soluble fraction of a 95% ethanol extract with cytotoxic activity was phytochemically investigated. A new tetranorlanostane triterpenoid ( 1 ) and a new ergosteroid ( 4 ), together with two known lanostanes ( 2 and 3 ) and six known Δ ²² -ergosteroids ( 5 − 10 ), were isolated. Notably, compound 1 possesses a new 24,25,26,27-tetranorlanostane carbon skeleton. All isolates, except compound 8 , were tested for cytotoxic activities using a human breast cancer cell line (MDA-MB-231) and a human gastric cancer cell line (HGC-27) in vitro. The new nortriterpenoid ( 1 ) exhibited no obvious cytotoxicity, while the known triterpenoid ( 3 ) showed significant cytotoxicity against MDA-MB-231 and HGC-27 cells. Compound 4 exhibited the highest cytotoxicity against MDA-MB-231 and HGC-27 cells, with IC ₅₀ values of 11.33 ± 2.18 and 12.28 ± 3.64 μ M, respectively. Furthermore, Hoechst fluorescence 33342 staining and Western blot tests demonstrated that compound 4 induced apoptosis in MDA-MB-231 cells by downregulating procaspase-3 expression and upregulating the expression ratio of Bax/Bcl-2.