The primary goal of AtlantOS is to transition a currently poorly coordinated set of ocean observing activities towards a sustainable, efficient and fit-for-purpose Integrated Atlantic Ocean Observing System (IAOOS) by defining requirements and system design, improving the readiness of observing networks and data systems, and engaging Atlantic stakeholders; and leaving a legacy and strengthened contribution to the Global Ocean Observing System (GOOS) and the Global Earth Observing System of Systems (GEOSS). AtlantOS will fill existing gaps in the in situ observing system and ensure that data are easily accessible and usable. AtlantOS will demonstrate the usefulness of integrating in situ and satellite-based observations that observe the earth to inform a wide range of sectors using the Copernicus Marine Monitoring Service and the European Marine Observation and Data Network and connect them with similar activities in the Atlantic. AtlantOS will support activities to share, integrate and standardise in situ observations, reduce costs through network optimisation and deployment of new technologies and increase the competitiveness of European industries, particularly small and medium-sized enterprises in the maritime sector.AtlantOS will promote innovation, documentation and exploitation of innovative observing systems.All AtlantOS work packages will strengthen transatlantic collaboration, through close interaction with institutions in Canada, the United States and the South Atlantic region. AtlantOS will develop a results-oriented dialogue with stakeholder communities to enable a meaningful exchange between the products and services that IAOOS can offer and the demands and needs of the stakeholder communities.Finally, AtlantOS will establish a structured dialogue with funding agencies, including the European Commission, the United States, Canada and other countries to ensure the sustainability and appropriate growth of IAOOS.