The increase in popularity of citizen observatories (COs) over the last decade has come with increased participation and large amounts of collected data. This has evidenced the challenges faced by COs associated with low interoperability, low levels of data validation and low technological capacity. COs need to ensure that data of various formats and in large amounts is efficiently collected, identified, validated, analysed and stored. Moreover, interoperability must be ensured if data is to be shared across COs, organisations or reporting bodies. The overarching vision of the Cos4Cloud project (‘Co-designed citizen observatories for the EOS-Cloud’) is to integrate citizen science infrastructures into the European Open Science Cloud (EOSC) through the co-design of innovative services and tools to solve challenges faced by COs, while bringing citizen science closer to the scientific community and society and providing new data sources. This document describes some of the challenges faced by COs mentioned above and how the services developed by Cos4Cloud have responded to this need while integrating citizen science infrastructures into the EOSC in a groundbreaking manner and contributing to the ambitions of the EU’s open science policy. The document also argues in favour of sustaining the services for COs and recommends actions for doing so. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement no. 863463.