A new quassinoid, yadanziolide S (1), was isolated from the seeds of the traditional Chinese medicinal herb, Brucea javanica, along with ten known compounds, flazin, bruceine D, yadanziolide B, bruceoside A, yadanziolide S, yadanzigan, glycerol 1,3-bisoleate, azelaic acid, (+/-)-8-hydroxyhexadecanoic acid, and vanillin. Compound 1 is the first quassinoid to have been isolated from B. javanica without a methyleneoxy bridge between C-8 and C-13, and its structure was determined using spectroscopic methods, and confirmed by single-crystal X-ray diffraction. All isolates were evaluated for their potential to induce human promyelocytic leukemia (HL-60) cell differentiation, and to inhibit cyclooxygenase-1 (COX-1), -2 (COX-2), and 7,12-dimethylbenz[a]anthracene (DMBA)-induced lesions in a mouse mammary organ culture model.