The first chemical study on the seeds of Caesalpinia sappan Linn. led to isolation of 11 cassane-type diterpenes, named phanginin A–K (1–11). The skeleton present in compounds 1–8 is rather unusual, consisting of a cassane-type diterpene with an ether bridge between C-19/C-20 in compounds 1–6 and C-11/C-20 in compounds 7 and 8. Their structures were elucidated on the basis of spectroscopic techniques. In addition, the X-ray structure of phanginin A (1) is reported. Only phanginin I (9) exhibited cytotoxic effect against KB cell line with IC50 value of 4.4 μg/ml.