Aristotle University of Thessaloniki (AUTH) organized a 2-day national training of trainers, within the project MARLITER “Improved online public access to environmental monitoring data and data tools for the Black Sea Basin supporting cooperation in the reduction of marine litter” with funding from the Joint Operational Program “Black Sea Basin 2014-2020”, (European Neighbourhood & Partnership Instrument – ENPI). The training took place online on Monday 2 and Tuesday 3 November, at a virtual meeting room via Zoom Platform. At the training of trainers, experts related to environmental education, management of cooperation projects, protection of aquatic ecosystems, citizen science applications, conservation and management of vulnerable species and habitats, originated from universities, sectoral agencies, NGOs and training centres, provided their knowledge and expertise on different aspects of the marine litter issue. The training of trainers was attended by teachers, students, youth volunteers, representatives of sectoral agencies and businesses, representatives of NGOs and groups of citizens. The training of trainers included presentations and interactive discussions on application of environmental education actions, opportunities for public involvement in collaborative strategies, enhancement of citizen science, implementation of beach litter monitoring and clean-up campaigns, etc. The experts offered to the participants data, background information and tools so as to increase involvement of local communities in prevention of pollution and mitigation activities, promote the development and application of management and monitoring strategies and enhance knowledge regarding environmental protection and joint reduction of marine litter. The participants gained valuable experience and committed to further transfer and disseminate the acquired knowledge to larger audience.