Purpose: The paper aims to elaborate on long-term throughput forecasts in Small and Medium-Sized ports (SMPs). The research problem relates to determining a method that is relevant for the long-term transhipment forecasting in SMPs. Design/Methodology/Approach: Research was applied to the Polish ports system consisting of three major ports (Gdansk, Gdynia, and Świnoujście) and the minor port of Szczecin. Forecasts for the cargo groups in the major ports were produced using regression models where the model parameters were estimated with the Ordinary Least Squares Method. The obtained forecasts of the throughput of cargo groups in major Polish seaports were converted into dynamics indices. The resulting matrices of the cargo throughput dynamics indices established for the leading Polish seaports were used to prepare cargo throughput forecasts for the port of Szczecin. Findings: Elaborated throughput forecasts indicate that, in the future, the port in Szczecin will retain its universal character, and a moderate increase in the cargo volumes confirms that it will serve as a complementary port to the major ports in the range. The forecasted modal split of hinterland transport indicates the increasing importance of road transport, and a decreased rail and inland waterway transport to/from the hinterland. That is likely due to a lower cargo throughput and difficulties in organizing the rail and inland waterway corridors based on smaller freight volumes. Practical implications: The study provides a practical tool for long-term throughput and hinterland traffic forecasting in SMPs. It is addressed to port authorities as the method may be used in decision-making on capacity expansion and academics exploring the phenomenon of predictions in maritime transport. Originality/Values: The novel method of demand forecasting in SMPs includes relations between major and minor ports in the range and multi-staged validation of results. Research advances studies on the dynamics of multi-port systems.