The phycotoxin dinophysistoxins are widely distributed in the global marine environments and potentially threaten marine organisms and human health. The mechanism of the dinophysistoxin toxicity in inhibiting the growth of microalgae is less well understood. In this study, effects of the dissolved dinophysistoxin-1 (DTX1) on the growth, pigment contents, PSII photosynthetic efficiency, oxidative stress response and cell cycle of the marine microalga Isochrysis galbana were investigated. Growth of I. galbana was significantly inhibited by DTX1 with 0.6–1.5 μmol L⁻¹ in a 96-h batch culture, corresponding the 96 h-EC₅₀ of DTX1 at 0.835 μmol L⁻¹. The maximum quantum yield of PSII (F_v/F_m), and light utilization efficiency (α) were obviously reduced by DTX1 at 1.5 μmol L⁻¹ during 96-h exposure. Contents of most of pigments were generally reduced by DTX1 with a dose-depend pattern in microalgal cells except for diatoxanthin. The ROS levels were increased by DTX1 with 0.6–1.5 μmol L⁻¹ after 72-h exposure, while the contents or activities of MDA, GSH, SOD and CAT were significantly increased by DTX1 at 1.5 μmol L⁻¹ at 96 h. The inhibitory effect of DTX1 on the growth of I. galbana was mainly caused by the production of ROS in the cells. Cell cycle analysis showed that the I. galbana cell cycle was arrested by DTX1 at G2/M phase. This study enhances the understanding of the chemical ecology effects of DTX1 on marine microalgae and also provides fundamental data for deriving water quality criteria of DSTs for marine organisms.