A bioassay-guided fractionation and chemical investigation of the stem bark of Premna tomentosa resulted in the isolation and characterization of four new icetexane diterpenes (1-4), along with the known compounds coniferaldehyde (5), syringaldehyde (6), lupeol (7), betulin (8), and 2-(4-methoxyphenyl)-2-butanone (9). Their structures were established on the basis of extensive spectroscopic (IR, MS, 2D NMR) data analysis and by comparison with the spectroscopic data reported in the literature. The new compounds exhibited diverse functionalities on a common icetexane diterpene skeleton. In addition, cytotoxic activities of the icetexanes (1-3) were evaluated by determining their inhibitory effects on the human cancer cell lines (MCF-7, HT-29, Hep-G2, A-431, and A-549). Compounds 1 and 3 showed selective inhibitory activity against MCF-7 (15.96microg/mL and 15.84microg/mL) and HT-29 cell lines (16.21microg/mL and 14.57microg/mL), respectively.