Abstract Hypoxia, defined as dissolved oxygen falls below 2.8 mg O 2 L − 1 , affects several 100,000 km 2 of marine water worldwide. Previous studies have shown that hypoxia results in aberrant behaviors of benthic fauna, collapse of fisheries, and major changes in the structure and trophodynamics of marine ecosystems. Nevertheless, it is not known whether, and if so how, hypoxia can affect the phytoplankton, which accounts for the major part of the primary productivity of marine ecosystems. The present study investigated the effects of hypoxia on the growth of the diatom Skeletonema costatum . Diatoms were cultured at three levels of oxygen (i.e. 7.0, 2.0, 0.5 mg O 2 L − 1 ) for 14 days, and the biological endpoints (i.e. cell size, cell density, optical density, chlorophyll a concentration, adenylate energy charge) were measured at 0, 1, 3, 5, 7, and 14 days. Our results demonstrated that all metrics and the calculated specific growth rate over the 14 day experimental period were negatively affected by 0.5 mg O 2 L − 1 . Results of this study, for the first time, provided the evidence and important insight of the impacts of hypoxia on the functions of phytoplankton in marine ecosystems.