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The fungal genus <i>Pyrenochaetopsis</i> is commonly found in soil, terrestrial, and marine environments, however, has received little attention as a source of bioactive secondary metabolites so far. In a recent work, we reported the isolation and characterization of three new anticancer decalinoyltetramic acid derivatives, pyrenosetins A–C, from the Baltic <i>Fucus vesiculosus</i>-derived endophytic fungus <i>Pyrenochaetopsis</i> sp. FVE-001. Herein we report a new pentacyclic decalinoylspirotetramic acid derivative, pyrenosetin D (<b>1</b>), along with two known decalin derivatives wakodecalines A (<b>2</b>) and B (<b>3</b>) from another endophytic strain <i>Pyrenochaetopsis</i> FVE-087 isolated from the same seaweed and showed anticancer activity in initial screenings. The chemical structures of the purified compounds were elucidated by comprehensive analysis of HR-ESIMS, FT-IR, [α]_D, 1D and 2D NMR data coupled with DFT calculations of NMR parameters and optical rotation. Compounds <b>1–3</b> were evaluated for their anticancer and toxic potentials against the human malignant melanoma cell line (A-375) and the non-cancerous keratinocyte cell line (HaCaT). Pyrenosetin D (<b>1</b>) showed toxicity towards both A-375 and HaCaT cells with IC₅₀ values of 77.5 and 39.3 μM, respectively, while <b>2</b> and <b>3</b> were inactive. This is the third chemical study performed on the fungal genus <i>Pyrenochaetopsis</i> and the first report of a pentacyclic decalin ring system from the fungal genus <i>Pyrenochaetopsis</i>.