Bioassay-directed fractionation of an extract from the grain-based culture of the coprophilous fungus Penicillium sp. G1-a14 led to the isolation of a new eremophilane-type sesquiterpene, 3R,6R-dihydroxy-9,7(11)-dien-8-oxoeremophilane (1), along with three known analogues, namely, isopetasol (2), sporogen AO-1 (3), and dihydrosporogen AO-1 (4). The structure of 1 was elucidated using 1D and 2D NMR and single-crystal X-ray diffraction. Assignment of absolute configuration at the stereogenic centers of 1 was achieved using ECD spectroscopy combined with time-dependent density functional theory calculations. Sporogen AO-1 (3) and dihydrosporogen AO-1 (4) caused significant inhibition of radicle growth against Amaranthus hypochondriacus (IC50 = 0.17 mM for both compounds) and Echinochloa crus-galli (IC50 = 0.17 and 0.30 mM, respectively).