Fourteen new cassane diterpenoids, caesalminaxins A-L (1-14), and three known compounds were isolated from the seeds of Caesalpinia minax. Among the new diterpenoids, compounds 3 and 4 possess a rare spiro C/D ring system. The C-16 epimeric mixture 1/2 has an unprecedented carbon skeleton, presumably derived from 3 by cleavage of the C-13-C-14 bond. Compound 5 is the first example of a cassane diterpenoid with a spiro A/B ring system. The structures of the compounds were elucidated on the basis of 1D and 2D NMR analysis, and the absolute configurations of 3, 4, 9, and 11 were determined by single-crystal X-ray crystallography. Biosynthesis pathways for 1/2, 3, and 5 are postulated. Compounds 4, 8, and the known bonducellpin D exhibited moderate activity against four tested human cancer cell lines, HepG-2, K562, HeLa, and Du145.