In continuation of studies on the marine fungal isolate Penicillium thomii KMM 4675, 2(S)-acetamido-3-phenylpropylacetate (1)—a compound previously obtained synthetically—was isolated for the first time from this fungus. The fungus was cultivated on standard rice medium for 21 days at room temperature. The metabolite was extracted with ethyl acetate, and purified using silica gel column chromatography and high-performance liquid chromatography (HPLC). The molecular formula of 1 was determined as C₁₃H₁₇NO₃ by high-resolution electrospray-ionization mass spectrometry (HR-ESI-MS), and its structure was elucidated via proton magnetic resonance (PMR), carbon-13 nuclear magnetic resonance (¹³C NMR), heteronuclear single-quantum correlation (HSQC), correlation spectroscopy (COSY), and heteronuclear multiple-bond correlation (HMBC) spectra. The S configuration at the asymmetric C-2 was confirmed by comparing the specific rotation of 1 with literature data for (R)- and (S)-isomers. Cytotoxicity assays using the MTT method showed that compound 1 did not exhibit cytotoxicity against normal human prostate cell lines PNT-2 and RWPE-1 at concentrations up to 100 μM after 72 hours of treatment.