State authorities may influence the development of seaports by employing the tools of national maritime policy. On the one hand, seaports contribute to the socioeconomic development of coastal regions; on the other, they have a significant impact on foreign trade turnover. The aim of this study is to identify the major factors that have influenced the development of Poland’s seaports in the context of the country’s maritime policy. The paper examines and explains the development and transitions of major Polish seaports such as Gdansk, Gdynia, Szczecin, and Swinoujcie. In order to identify the state of the port economy the authors used public statistics and data analysis. Furthermore, they created a model of comanagement of major seaports and presented a qualitative and quantitative comparison of the development of major seaports from 2005 to 2019. It was discovered that port turnover increased, but in various ways in each of the analysed ports. The government of Poland, acting in a dual role as the coordinator of national maritime policy and the majority owner of seaports, was the most powerful decision-maker in the port economy. However initiatives to implement sustainable principles in seaports have gradually emerged through bottom-up activities of port authorities supported by local and regional authorities.