Neoboutomannin A (1), a new degraded diterpenoid monomer, and 3α-acetyl-14α-hydroxytaraxan-19,28-olide (2), a new triterpenoid derivative, have been isolated from the stem bark of Neoboutonia macrocalyx Benth (Euphorbiaceae), together with the known compounds, 3-acetyl aleuritolic acid (3), 3β-acetoxy oleanolic acid (4), oleanolic acid (5), 3,3,4-tri-O-methylellagic acid (6), sitosterol 3-O-β-d-glucopyranoside (7) and sitosterol (8). Their structures were elucidated on the basis of spectral data and comparison with those present in the literature. The methanol extract of the stem bark of N. macrocalyx showed high toxicity to brine shrimp nauplii (LC50 = 0.6 ± 0.05 μg/mL) and low antifungal activity on Candida albicans and Mucus miehei. Compound 3 exhibited moderate toxicity to brine shrimp (LC50 = 10.0 ± 0.9 μg/mL) and compound 2 showed low antifungal activity.