The population structure of the non-indigenous calanoid copepod Pseudodiaptomus marinus (Sato, 1913) in the Belgian part of the North Sea (BPNS) is reported for the first time. Detailed P. marinus abundance data including sex and age class of the individuals was gathered on a monthly basis from February 2015 to February 2016 at six sites within the BPNS and Belgian harbors. Relevant environmental variables were analysed to identify potential drivers explaining the population structure of P. marinus within the BPNS. The abundances found were unexpectedly high, with peak densities of up to 560 ± 163 ind.m -3 . Even though P. marinus was found in all stations sampled, large spatial and temporal differences were found in the abundance of this species. P. marinus population structure was best explained by water temperature and chlorophyll a concentrations, while salinity and concentrations of dissolved inorganic nitrogen did not influence the distribution. The reported high abundances of the species, especially in the harbor of Zeebrugge, together with the high relative abundances of copepodites indicate that the species is able to reproduce within the BPNS and Belgian harbors, possibly leading to an established, permanent population. It is crucial to study the distribution of this species for a longer period in order to determine the possible establishment of this species in the BPNS and consequences for local planktonic populations.