Thirteen new dolabrane-type diterpenoids, koilodenoids A-M (1-13), including a chlorinated congener (2), along with six known analogues, were isolated from Koilodepas hainanense. The structures were determined by analysis of spectroscopic data, ECD spectra, and X-ray crystallographic studies. The absolute configuration of C-15 in the 15,16-diol unit of compounds 4 and 5 was established by using the dimolybdenum tetraacetate [Mo2(AcO)4]-induced ECD method. Compounds 4, 7, 16, 17, and 19 showed moderate to significant immunosuppressive activities against the proliferation of T and B lymphocytes in vitro, with compound 16 being the most potent (IC50 0.86 and 0.29 μM, respectively).