The toxic dinoflagellate, Stoeckeria algicida, belongs to the family Thoracosphaeraceae. It invades fish cells, causing high fish mortality, and also kills other marine microalgae.. algicida was first found in Masan Bay, Korea, in 2004. The morphology and ecology of S. algicida have been reported, and it has been identified as an invasive microalgae. Its vegetative and biflagellate cells are oval, and measure 7.3-15.9 mm (mean 11.6) and 2.7-12.2 mm (mean 7.3), respectively. Based on morphological and genealogical analyses, a previous study suggested that it could be a new species in a new genus. Grazing coefficients of up to 0.142 min^-1 have been reported for S. algicida on Heterosigma akashiwo, suggesting that S. algicida grazing can have a considerable impact on H. akashiwo populations. Morphological identification of S. algicida is difficult because of its small size, which has delayed research progress. Therefore, research on S. algicida in the China Sea has been limited. Recently, the development of high-throughput next generation sequencing (NGS) technology has increased nano- and picophytoplankton research output. To identify S. algicida and its distribution in Liaodong Bay, we used the 18S rDNA V4 region as the target gene to design the primer V4(F/R), and employed NGS technology to investigate nano- and picophytoplankton diversity in Liaodong Bay. The results revealed that S. algicida was present in Liaodong Bay in every season except spring. Temperature was a significant factor in S. algicida reproduction. S. algicida was not dominant in the phytoplankton community, but its density was higher in summer (2.753×10³ cells/L). S. algicida density was high on the east and west coasts of Liaodong Bay, where the risk of disastrous ecological impacts is higher. In the study, S. algicida was found and reported in Liaodong Bay, China, S. algicida could be classified as a newly naturalized species based on its distribution in Liaodong Bay. Although there are no records of red tides caused by S. algicida, measures should be taken to prevent such an occurrence, rather than mitigating the effects should a red tide occur. Furthermore, monitoring and management of toxic microalgae in aquacultural areas should be increased and a red tide early warning system should be established to avoid disasters in fisheries ecosystems.