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 Δ²²-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 IC₅₀ values of 11.33 ± 2.18 and 12.28 ± 3.64 μM, 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.