CS3MESH4EOSC implements a service for the European Open Science Cloud (EOSC) with a built-in sustainability model using the on-premise service delivery by utilizing existing key technology enablers: Open Cloud Mesh (OCM) standardized protocol and EduGAIN service. It consolidates and integrates the existing application ecosystem by promoting vendor-neutral APIs and protocols following the open-source strategy for delivering services — a platform for a thriving application ecosystem in EOSC. CS3MESH4EOSC empowers service providers in delivering state-of-the-art, connected infrastructure to boost effective scientific collaboration across the entire federation and data sharing according to FAIR principles.The project delivers the core of a scientific and educational infrastructure for cloud storage services in Europe through a lightweight federation of existing sync/share services and integration with multidisciplinary application workflows.