Casearborin c (1), syringic acid (2), ent-3β-hydroxy-(-)-13-epi-manoyl oxide (3), ent-(-)-13-epi-manoyl oxide (4), ent-(-)-kaur-16-en-19-oic acid (5), and γ-sitosterol (6) were isolated from Casearia corymbosa stem bark. Only casearborin c showed cytotoxic activity on HeLa and SiHa cancer cell lines. This work contributes to the description of three compounds (1-3) newly isolated from C. corymbosa and highlights that casearborin c, a clerodane-type diterpene, is responsible for the cytotoxic activity shown in the original methanol extract of this species.