Chemical investigation of the antimalarial medicinal plant Clerodendrum polycephalum led to the isolation of five new diterpenoids, including ajugarins VII-X (1-4) and teuvincenone K (5), along with four known compounds, namely, 12,16-epoxy-6,11,14,17-tetrahydroxy-17(15 → 16)-abeo-5,8,11,13,15-abietapentaen-7-one (6), methyl pheophorbide A (7), loliolide (8), and acacetin (9). The chemical structures of the new compounds were elucidated using NMR spectroscopy, mass spectrometry, circular dichroism, as well as density functional theory calculations. All compounds were evaluated for in vitro activity against Plasmodium falciparum 3D7 malaria parasites with methyl pheophorbide A (7) showing the strongest activity (IC50 4.49 μM). Subsequent in vivo testing in a Plasmodium berghei chemosuppression model showed that compound 7 significantly attenuated peripheral blood parasitemia, leading to 79% and 87% chemosuppression following oral doses at 10 and 20 mg/kg, respectively.