Water temperature can affect the metabolic rates and biological activity of aquatic organisms. The pumpkinseed (Lepomis gibbosus) is a successful invasive fish species found chiefly in southern parts of Europe; populations from northern latitudes are characterised by lower levels of invasive potential. This study aimed to analyse morphological variability in populations from variable latitudes throughout Europe for comparison with a population from a habitat with artificially increased water temperature and to compare this population with a population relocated from a heated habitat to a habitat with a temperature typical for the given latitude. We measured twenty-six morphometric characters, which were subsequently statistically analysed. The species was characterised by high morphological variability among all examined populations. A population from Portugal was clearly separated from the other populations analysed, indicating a different body shape. For the two Polish populations, thirteen characters were statistically different, and one had biological importance. The population from the artificially heated habitat achieved larger sizes and more robust body proportions than the relocated population.