The world-wide crisis caused by the Coronavirus disease 2019 (COVID-19) pandemic had a significant impact on the global economy functioning and the sustainable development of supply chains. The changes also affected seaports being the key links of maritime supply chains. The purpose of the research study described in this article was to identify the sources and kinds of disruptions observed in various maritime supply chains as a result of the COVID-19 pandemic and their impact on the operations of various types of seaport terminals, namely those serving bulk (universal, specialised) and general cargoes (universal, specialised). An additional purpose was to identify the dependencies between the type of terminal and its main function, and the tactical decisions adopted by the particular terminals. The research was carried out using the multiple-case study method. The study covered some selected port terminals functioning in Polish seaports (Gdańsk, Szczecin, Świnoujście), applying direct, semi-structured in-depth interviews. The analysis of the results was carried out using the inductive reasoning method. The research study has shown that as a result of the COVID-19 pandemic some maritime supply chains ceased to exist, some of them were operating with decreased cargo volumes, while in other cases the transshipment volumes actually rose during the pandemic. Among terminal operators’ tactical responses to disruptions in maritime supply chains, there were pro-active and adaptive measures. Pro-active (offensive) measures included actions taken by an enterprise in order to engage in new maritime supply chains, and even participating in establishing new maritime chains in response to limitations caused by the pandemic. Adaptive (defensive) measures covered actions taken by the port terminals as a consequence of changes in the existing maritime supply chains, caused by the pandemic in the port’s foreland or hinterland. The research study results revealed that the terminals extent of engagement and tactical decisions related to the pandemic were depended on the type of terminal (universal or specialised) and its main function played within a supply chain.