As part of our research on bioactive metabolites from marine-derived fungi, we chemically investigated the fungus Diaporthaceae sp. PSU-SP2/4 isolated from a marine sponge collected from Pak Meng beach, Sikao, Trang province, Thailand. Purification of the mycelial EtOAc extract by different chromatographic techniques led to the isolation of one new pentacyclic lactam, diaporthalasin (1). In addition, the broth EtOAc extract yielded one new ethyl trihydroxytridecatrienoate derivative, diaporthacol (2), which was isolated as its triacetate derivative (3), together with five known compounds, R-mevalonolactone (4), dothiorelone C (5), (4S,7S,13S)-4,7-dihydroxy-1,3-tetradeca-1,5-dienolide (6), 4b-acetoxy-9b,10b,15a-trihydroxyprobotrydial (7) and O-methyldihydrobotrydial (8). The structures were established by spectroscopic techniques. Those compounds isolated in sufficient quantity were evaluated for antibacterial and cytotoxic activities. Compound 1 displayed significant antibacterial activity against both Staphylococcus aureus and methicillin-resistant S. aureus with equal MIC values of 2 mg/mL.