<jats:title>Abstract</jats:title><jats:p>Five new diketopiperazines, prenylcyclotryprostatin B (<jats:bold>1</jats:bold>), 20‐hydroxycyclotryprostatin B (<jats:bold>2</jats:bold>), 9‐hydroxyfumitremorgin C (<jats:bold>3</jats:bold>), 6‐hydroxytryprostatin B (<jats:bold>4</jats:bold>), and spirogliotoxin (<jats:bold>5</jats:bold>), were isolated from the marine‐derived fungus <jats:italic>Aspergillus fumigatus</jats:italic> YK‐7, along with nine known compounds, <jats:bold>6</jats:bold>–<jats:bold>14</jats:bold>. Their structures were elucidated by spectroscopic methods, and their antiproliferative effects on human leukemic monocyte lymphoma U937 and human prostate cancer PC‐3 cell lines were assessed <jats:italic>in vitro.</jats:italic> Compounds <jats:bold>10, 12</jats:bold>, and <jats:bold>13</jats:bold> exhibited significant cell growth‐inhibitory activities against U937 cell line, with the <jats:italic>IC</jats:italic><jats:sub>50</jats:sub> values of 1.8, 0.2, and 0.5 μ<jats:sc>M</jats:sc>, respectively.