Abstract: As unicellular holozooplankton, polycystine radiolarians play critical roles in ocean eco-system and biogeochemical cycles. However, the knowledge of sea surface radiolarian biodiversity and biogeography is deficient. This research samples are obtained from 46 stations during spring 2017 and winter 2018 in North Indian Ocean. Radiolarian abundance in North Indian Ocean is influenced by ocean currents and responds to thermohaline variations. The species diversity of radiolarians peaked in winter, particularly in the southern Bay of Bengal and Andaman Sea. Taxonomically, Spumellaria dominated the assemblage, followed by Nassellaria and Collodaria. Collodaria exhibited the strongest affinity for warm, low-salinity waters. Tetrapyle spp. and Buccinosphaera invaginata were ubiquitous dominant species, while distinct seasonal indicator species were identified: Didymocyrtis tetrathalamus, Botryocyrtis scutum and Spongaster tetras were associated with nutrient-poor, strongly stratified waters in spring, whereas Tetrapyle octacantha and Spongosphaera streptacantha indicated nutrient-rich, weakly stratified conditions in winter. Multivariate analyses confirmed these seasonal distinctions, showing that the similar hydrological regimes foster shared species composition and clustering patterns. This study elucidates the spatial and seasonal dynamics of radiolarian communities in surface water, driven primarily by temperature–salinity gradients and inorganic nutrients, thereby advancing our understanding of zooplankton biogeography and its response to climate change and regional environmental forcing.