One new methoxylated flavonoid derivative, plectranmicin (1) and one new monoterpene derivative, plectranmicinol (2), together with seven known compounds including 5-hydroxy-3,7,2′,4′-tetramethoxyflavone (3); 5,7-dihydroxy-3,2′,4′-trimethoxyflavone (4); 7-hydroxy-5,6,4′-trimethoxyflavone (5); 3-epi-betulinic acid (6); 3-O-β-D-glucopyranosyl stigmasterol (7); β-sitosterol (8) and 4-epi-fridelin (9) were isolated from the whole plant of Plectranthus glandulosus Hook. The structures of these compounds were established on the basis of extensive analysis of their spectroscopic data, as well as one and two-dimensional NMR experiments. The absolute configuration of compound 2 was established by X-ray crystallographic analysis. Although compounds 3 and 4 have been previously reported as synthetic compounds, it is the first time that these compounds are isolated from a natural source. Compounds 1–5 were evaluated for their antioxidant activity using the scavenging activity of the free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH°) and the Ferric Reduction Antioxidant Power (FRAP). The results obtained showed that compounds 1 and 2 displayed significant antioxidant activity with IC50 values of 0.208 and 0.322 μM/mL; 0.185 and 0.339 μM/mL respectively whereas, the activity of the other was moderated. It is the first time that the antioxidant activity of these compounds are reported.