The introduction of harmful aquatic organisms and pathogens (HAOP) via the ballast water of ships can endanger human health, the environment, economic resource-related systems, as well as impair biological diversity. Early warning systems (EWS) are highly effective instruments for mitigating risks and enhancing readiness in the face of potential disasters, including those resulting from HAOP. The establishment of a regional EWS for the identification and notification of harmful organisms is a complex marine policy process involving communication with stakeholders, technical development, engagement of regional knowledge, and, ultimately, harmonization between key authorities and Participating States. The purpose of EWS is to promptly communicate the findings of HAOP to the appropriate stakeholders, such as government agencies and commercial companies. The paper elucidates in detail the criteria and procedure for developing a regionally standardized EWS, including its mechanism, the technical information platform that is required, and the obligations of EWS data providers. It also discusses the decision-making process for issuing alerts and identifies the intended recipients of the warning signals. The system has been operational in the Baltic Sea region for over two years, and the knowledge and insights gathered during its development may prove beneficial in other areas as well.