Seaweed extracts are widely used in agriculture as ecological focus substances applied to improve crop growth and quality. One of the primary benefits they bring is increased effectiveness of fruit setting as well as improved stress tolerance, essential for warm-climate crops cultivated in the nonoptimal environmental conditions of Northern and Central Europe. The aim of this study was a preliminary investigation of any genotype-dependent reaction of eggplant cultivars (Solanum melongena) to application of a standardized extract of the seaweed Ascophyllum nodosum (Göemar BM-86) under field conditions in Poland. The only statistically relevant result of this biostimulant was shown for cultivar ‘Flavine’ F1, where it positively affected the early crop yield and the number of fruits per plant. Fruit quality attributes, including antioxidant activity, as well as selected mineral contents, increased as an effect of biostimulant spraying. This reaction was specific for the cultivars investigated, and it was confirmed by significant differences in the main effects between biostimulant and control treatments for almost all the properties measured. The use of this A. nodosum extract suggested that there could be an improvement in fruit yield and quality in selected eggplant cultivars under field conditions in the temperate climatic zone.