This study examines the port planning dynamics of the neighbouring ports of Koper (Slovenia) and Trieste (Italy) since the Second World War. Through an analysis of port-related planning documents, it offers insights into how geographical proximity, divergent geopolitical circumstances, and a unique border context have shaped the development trajectories of these two ports. We argue that their spatial planning is somewhat idiosyncratic because the presence of a border did not necessarily condition unrelated planning efforts and vice versa. Max Fabiani was the only urban planner to propose a joint development plan for both port cities in the immediate postwar period. The port and local authorities pursued separate planning paths for the two port cities after the Yugoslav–Italian border was established in 1954. As the border became more permeable, a certain relatedness can be seen in the spatial planning of both ports. The port authorities worked toward a unified port system at the turn of the millennium, but the persistence of phantom borders prevented this. In recent decades of European borderless integration, there are fewer obstacles to planning cooperation. Currently, both ports are planning similar development, such as the expansion of container terminals, which is leading to their increased competition.