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From the rice fermentation product of a new record fungus, <i>Neohelicosporium griseum</i>, two new polyketides, neogrisphenol A (<b>1</b>) and neogrisphenol B (<b>2</b>), one new isochroman-1-one, (<i>S</i>)-6-hydroxy-7-methoxy-3,5-dimethylisochroman-1-one (<b>3</b>), and four known compounds (<b>4</b>–<b>7</b>) were isolated. Their structures were determined using 1D- and 2D-NMR, mass spectrometry, and chemical calculations. The C-3~C-2′ polymerization mode between the two <i>α</i>-naphthalenone derivative moieties is uncommon in compounds <b>1</b> and <b>2</b>. Meanwhile, compounds <b>1</b>–<b>2</b> and <b>5</b> exhibited antibacterial activity against <i>Bacillus subtilis</i>, <i>Clostridium perfringens</i>, <i>Staphylococcus aureus</i>, and <i>Staphylococcus aureus</i>, with MIC values ranging between 16 and 31 µg/mL. In addition, compound <b>5</b> showed antifungal activity against <i>Sclerotinia sclerotiorum</i> and <i>Phytophthora nicotianae var. nicotianae</i>, with respective IC₅₀ values of 88.14 ± 2.21 µg/mL and 52.36 ± 1.38 µg/mL. Compound <b>1</b> showed significant cytotoxicity against A2780, PC-3, and MBA-MD-231 cell lines with respective IC₅₀ values of 3.20, 10.68, and 16.30 µM, and the cytotoxicity against A2780 cells was even higher than that of cisplatin (CDDP). With an IC₅₀ value of 10.13 µM, compound <b>2</b> also exhibited cytotoxicity against A2780. The in vitro results showed that compound <b>1</b> inhibited A2780 cell proliferation, induced apoptosis, and arrested the cell cycle at the S-phase in a concentration-dependent manner.