Seasonality of plankton communities reveals their diversity and plasticity in response to fluctuating environments. However, many marine environments, such as offshore islands, remain poorly studied in this respect. We investigated seasonal changes in the rich phytoplankton and zooplankton communities along the western coast of Sado Island, in the east central Sea of Japan, where there have been no detailed plankton studies for more than four decades. To understand environmental influences on seasonal plankton changes, we set three sampling stations 5 (St. A) and 1 km offshore (St. B), and on the coastline (St. C). Plankton sampling was conducted monthly and seasonally from 2022 to 2024. Based upon morphology, 49 phytoplankton and 199 zooplankton operational taxonomic units (OTUs) were identified. Diatoms, copepods and radiolarians were the most abundant and diverse groups. Among phytoplankton, diatoms (Thalassiosirales, Chaetoceros and Rhizosolenia) were dominant from spring to autumn, but dinoflagellates (Ceratium) were most common in winter. Guinardia, Rhizosolenia, and Ceratium were abundant at the near shore station (St. B) in summer, and at all stations in autumn, whereas they were uncommon during other seasons. Among zooplankton, copepods, copepod nauplii, and meroplankton (egg sp. 1 and mysis sp. 1) were dominant in winter and spring, whereas warm water species such as radiolarians (Aulosphaera trigonopa, Phyllostauridae and Acanthometron pellucidumn), cladocerans (Evadne spinifera and Penilia avirostris), tunicates (Dolioletta gegenbauri) and foraminifera (Rotaliida and Globigerinidae) were dominant during summer and autumn. The Tsushima Warm Current passes close to the island, likely affecting appearances of warm-water plankton. Zooplankton communities along the coastline clearly differ from those offshore. For example, large calanoid and cyclopoid copepods generally appear offshore, whereas small harpacticoid copepods are found along the coastline. Zooplankton exhibit diverse responses based on numbers of phytoplankton, such as Rhizosoleniaceae, Ceratium, and Guinardia, which co-occurred with dominant zooplankton OTUs.