Three new diterpenoids including two pimaranes (1 and 2) and a labdane (3) were isolated from the whole herb of Plectranthus ernstii. The structures of these compounds were determined as rel-15(zeta),16-epoxy-7alpha-hydroxypimar-8,14-ene (1) and rel-15(zeta),16-epoxy-7-oxopimar-8,14-ene (2), and compound 3 was elucidated as 1R,11S-dihydroxy-8R,13R-epoxylabd-14-ene on the basis of single-crystal X-ray structural analysis. Compound 1 exhibited moderate antistaphylococcal activity against a range of multidrug-resistant (MDR) and methicillin-resistant (MRSA) strains of Staphylococcus aureus with a minimum inhibitory concentration (MIC) of 32 microg/mL. All three diterpenes exhibited antimycobacterial activity against three strains of rapidly growing mycobacteria with MIC values ranging from 8 to 128 microg/mL.