Thirteen new dolabrane-type diterpenoids, koilodenoids A-M (<b>1</b>-<b>13</b>), including a chlorinated congener (<b>2</b>), along with six known analogues, were isolated from <i>Koilodepas hainanense</i>. 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 <b>4</b> and <b>5</b> was established by using the dimolybdenum tetraacetate [Mo<sub>2</sub>(AcO)<sub>4</sub>]-induced ECD method. Compounds <b>4</b>, <b>7</b>, <b>16</b>, <b>17</b>, and <b>19</b> showed moderate to significant immunosuppressive activities against the proliferation of T and B lymphocytes <i>in vitro</i>, with compound <b>16</b> being the most potent (IC<sub>50</sub> 0.86 and 0.29 μM, respectively).