Two new cassane diterpenes, caesaldecapes A-B (1–2), were isolated from the seeds of Caesalpinia decapetala. Their structures were elucidated by extensive spectroscopic methods, including NMR, MS, and HRESIMS. Compounds 1–2 are novel examples of rearranged cassane diterpenes obtained from C. decapetala. The two compounds were evaluated for their antiproliferative activity in vitro against four human cancer cell lines, and compound 1 showed selectively cytotoxic activity against KB cancer cell lines, with an IC50 value at 9.6μM.