From the mangrove plant Aegiceras comiculatum collected in South China an endophytic Streptomyces sp. was isolated. Following cultivation in a seawater-based medium, four new cyclopentenone derivatives, namely (5R) 3-amino-5-hydroxy-5-vinyl-2 cyclopenten-1-one (1), (5R) 5-hydroxy-3-[(methoxycarbonyl)amino]-5-vinyl-2-cyclopenten-1-one (2), (5R) 5 hydroxy-3-[[2-(4-hydroxyphenyl)ethyl]amino]-5-vinyl-2 cyclopenten-1-one (3), and 3-isobutylpropanamide-2 cyclopenten-1-one (4), were obtained from the fermentation broth. Their structures were elucidated by extensive spectroscopic (UV, IR, ESIMS, and 2D NMR) data analyses.