Maritime security, including the underwater, is of paramount importance to assure the smooth and safe transportation of goods and people to/from the EU ports, as well as to preserve uninterrupted operations of critical coastal-based/river-based/maritime infrastructures. Ships and maritime infrastructures need to be protected against all illegal acts through a) continuous 24/7 underwater security surveillance against potential intruders divers, hostile underwater vehicles and illicit/dangerous containers towed by ships, b) on-demand ships’ hulls scanning/inspection for the best possible detection/recognition/classification of strapped, attached, hidden or misplaced illicit (i.e. narcotics) and dangerous material, c) underwater inspections of ports’ entrances, seabed and docking places foundation for the best possible detection/recognition/classification of dangerous containers (i.e. CBRNE packages and IEDs) at these places, as well as to apply deterrent and response measures, in order to prevent/deter illicit acts and avoid undesired escalation of security incidents. To that end, the multinational and interdisciplinary UnderSec project team, consisting of experts in the maritime security and underwater security domain, research/academic institutions and technology, and End-Users from the Coast Guard, Borders Control, and Port Authorities sectors, will develop a modular/holistic approach Prototype System, consisting of multimodal sensors and robotic assets, for ships’, ports’ and maritime infrastructures’ Underwater Security Situational Awareness and Decision-Making Support, with relevant Response capabilities. Individual technologies will be tested, compared with each other and pre-evaluated in a controlled environment, while the modular components and the system as a whole will be tested and evaluated during real-life PUCs demonstrations in actual operational environments, resulting in the establishment of new knowledge, innovative solutions and advanced processes.