Twelve new cytochalasins, phomopchalasins D-O (<b>1</b>-<b>3</b>, <b>5</b>-<b>12</b>, and <b>14</b>), including one brominated (<b>2</b>) and two iodinated cytochalasins (<b>3</b> and <b>6</b>), together with six known analogues (<b>4</b>, <b>13</b>, and <b>15</b>-<b>18</b>) were isolated from the mangrove-derived fungus <i>Phomopsis</i> sp. QYM-13 treated with 3% NaBr or 3% KI in potato liquid medium. Their structures and absolute configurations were established by extensive spectroscopic analysis (1D and 2D NMR, HRESIMS), electronic circular dichroism calculations, and a single-crystal X-ray diffraction experiment. Compounds <b>3</b> and <b>6</b> represent the first iodinated cytochalasins. Compounds <b>2</b>, <b>15</b>, <b>17</b>, and <b>18</b> exhibited significant cytotoxicity against human cancer cell line MDA-MB-435 with IC<sub>50</sub> values ranging from 0.2 to 8.2 μM.