The contemporary security environment is characterized by the proliferation of hybrid threats that pose a particular challenge to the critical infrastructure of maritime states. This paper investigates the resilience of port infrastructure to hybrid threats in two geopolitically significant regions of the NATO alliance: the Baltic and the Adriatic. The research encompasses a comparative analysis of eight NATO member states – Estonia, Latvia, Lithuania, and Poland in the Baltic region, and Croatia, Slovenia, Montenegro, and Albania in the Adriatic region. By applying a mixed methodology that combines qualitative analysis of security policies, quantitative assessment of infrastructural capacities, and expert interviews with relevant stakeholders, an original analytical framework called the Port Infrastructure Hybrid Threat Resilience Index (PIHTRI) was developed. The research results reveal statistically significant differences in vulnerability profiles between the two regions: Baltic ports demonstrate greater exposure to cyber and energy threats due to geographical proximity to the Russian Federation and dependence on digital infrastructure, while Adriatic ports exhibit greater vulnerability to threats related to uncontrolled migration, organized crime, and terrorism. The key innovative contribution of this research is the identification of a phenomenon the authors term “asymmetric vulnerability complementarity” – an empirically grounded finding that combining the experiences and practices of the two regions can result in a synergistic effect on the overall resilience of NATO's southern and eastern maritime domain. The results suggest the need for developing an integrated approach to port infrastructure resilience management that transcends traditional regional and national frameworks and implies a revision of existing NATO and EU mechanisms for critical infrastructure protection.