Port-state jurisdiction has been used as a means of circumventing the inadequacies of enforcement on the high seas and of flag states’ ineffectiveness, but also the absence of international rules due to lack of consensus at the international level. Pressing and complex problems related to the global environment and global commons, such as depletion of fisheries, marine and atmosphere pollution, and climate change, and foot-dragging in the international community to effectively cooperate to tackle these problems have brought the concept of unilateral regulation of extraterritorial activities to the forefront. In this respect, the role of the port state, as a first point of contact for industries engaged in activities harmful to the global commons (i.e., fishing and shipping), is increasingly important. This article examines the scope and limits of port-state jurisdiction with respect to measures that may have an extraterritorial impact in the light of the law of the sea and international rules on jurisdiction. The aim of the article is to assess whether the practice of port states in exercising jurisdiction has contributed to developments regarding the exercise of (extraterritorial) jurisdiction as a regulatory tool for the protection of global commons. By identifying elements of current state practice regarding exercise of port-state jurisdiction, the article advances a framework for the most effective exercise of port-state jurisdiction for the protection of global commons with reference to the principle of common concern.