The chemical study on the seeds of Caesalpinia sappan led to the isolation of five new cassane diterpenoids, phanginins R-T (1-3) and caesalsappanins M and N (4 and 5), together with seven known compounds 6-12. Their structures were elucidated on the basis of NMR and HRESIMS analyses. The absolute configurations of compounds 1 and 4 were determined by the corresponding CD spectra. All the isolated compounds were tested for their cytotoxicity against ovarian cancer A2780 and HEY, gastric cancer AGS, and non-small cell lung cancer A549 cells. Compound 1 displayed significant toxicity against the four cell lines with the IC values of 9.9 ± 1.6 μM, 12.2 ± 6.5 μM, 5.3 ± 1.9 μM, and 12.3 ± 3.1 μM, respectively. Compound 1 induced G1 phase cell cycle arrest in A2780 cells. Furthermore, compound 1 dose-dependently induced A2780 cells apoptosis as evidenced by Hoechst 33342 staining, Annexin V positive cells, the up-regulated cleaved-PARP and the enhanced Bax/Bcl-2 ratio. What's more, compound 1 also promoted the expression of the tumor suppressor p53 protein. These findings indicate that cassane diterpenoids might have potential as anti-cancer agents, and further in vivo animal studies and structural modification investigation are needed.