| Summary: | Medicinal plants of the <i>Plectranthus</i> genus (Lamiaceae) are well known for their ethnomedicinal applications. <i>Plectranthus madagascariensis</i>, which is native to South Africa, is traditionally used in the treatment of respiratory conditions, scabies, and cutaneous wounds. The phytochemical studies of <i>P. madagascariensis</i> led to the isolation of five known royleanone abietanes, namely, 6β,7α-dihydroxyroyleanone (<b>1</b>), 7α-acetoxy-6β-hydroxyroyleanone (<b>2</b>), horminone (<b>3</b>), coleon U quinone (<b>4</b>), and carnosolon (<b>5</b>). The relative configuration of compound <b>2</b> was established by X-ray analysis. Compounds <b>1</b>–<b>4</b> showed antimycobacterial activity (Minimum inhibitory concentration for 90% inhibition, MIC<sub>90</sub> = 5.61–179.60 μM) against <i>Mycobacterium tuberculosis</i> H<sub>37</sub>Rv. Compound <b>4</b> and <b>5</b> showed comparable toxicity (Concentration for 50% inhibition, IC<sub>50</sub> 98.49 μM and 79.77 μM) to tamoxifen (IC<sub>50</sub> 22.00 μg/mL) against HaCaT cells. Compounds <b>1</b>–<b>5</b> showed antioxidant activity through single-electron transfer (SET) and/or hydrogen-atom transfer (HAT) with compound <b>5</b> being the most active antioxidant agent. Compounds <b>3</b> and <b>5</b> were isolated for the first time from <i>P. madagascariensis.</i> The observed results suggest <i>P. madagascariensis</i> as an important ethnomedicinal plant and as a promising source of diterpenoids with potential use in the treatment of tuberculosis and psoriasis.
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