The first record of Donacilla cornea (Poli) from Ukrainian beaches since 1960 is presented. The populations from the northern Black Sea were studied for their size and colour polymorphism. The maximum shell length was 26.05 mm, thus exceeding the previously reported values. The shell size differences between the populations and the similarity of shell shape indices were confirmed by one-way ANOVA. The negative allometry of height and width to length was confirmed by the t-test of linear regression slopes. Twelve patterns and seven background colours were recorded. Three patterns were observed for the first time. Significant differences in the frequency of colour pattern, and the similarity of pattern and shell colour between the studied populations were confirmed using Pearson’s χ². In the northern Black Sea populations white, non-streaked forms without radial pattern prevailed. The potential colour variation, estimated with ACE for each population, was close to the observed variation.