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A total of twenty-seven Rossa Mediterranea lactating goats, consisting of nine homozygous for strong alleles (<i>AA</i>), twelve heterozygous (<i>AF</i>) and six homozygous for weak alleles (<i>FF</i>) at the <i>CSN1S1</i> locus, were used to evaluate the effect of genotype, diet and genotype × diet interaction on goat milk traits and casein profile. The goats were used in a 3 × 3 factorial arrangement of treatments, with three genotypes (<i>AA</i>, <i>AF</i> and <i>FF</i>) and three different energy intake levels: high (H), medium (M) and low (L). The diets supplied a complete pelleted feed containing 65% of alfalfa hay, respectively, at 150%, 100% and 70% of the total energy requirements. Milk yield was significantly affected by the genotype and diet: Lower levels were found in <i>FF</i> goats than in <i>AA</i> and <i>AF</i> genotypes (673.7 vs. 934.5 and 879.8 d/g, respectively; <i>p =</i> 0.002) as well as in goats fed with the L diet (651.5 vs. 1041 and 852.9 g/d for H and M diet, respectively, <i>p <</i> 0.001). The genotype influenced the casein profile. Specifically, <i>AA</i> goat milk exhibited higher concentrations of total casein and α_s1-casein compared to <i>AF</i> and <i>FF</i> genotypes (for total casein and αs1-casein, respectively: 24.9 vs. 20.4 and 19.8 g/kg, <i>p =</i> 0.001; 7.2 vs. 3.7 and 0.7 g/kg, <i>p <</i> 0.001), while the <i>FF</i> genotype showed higher values for α_s2-casein concentrations compared to homozygous <i>AA</i> and heterozygous <i>AF</i> goats (3.1 vs. 2.4 and 2.5 g/kg, respectively, <i>p <</i> 0.001). A significant genotype x diet interaction occurred for αs2-casein levels (g/kg) (<i>p =</i> 0.034) and αs₁-casein yields (<i>p =</i> 0.027): The αs₂-casein level was not affected by the diet in <i>AA</i> goats, whereas it increased with energy intake in <i>AF</i> and <i>FF</i> genotypes. Conversely, the αs₁-casein yield gradually increased with energy intake in <i>AA</i> and <i>AF</i> groups, whereas the diet in <i>FF</i> goats did not modify it. The results demonstrated that high energy input, as well as the strong allele at the <i>CSN1S1</i> locus, enhanced milk production and casein concentrations. Furthermore, they confirmed the existence of an interaction between α_s1-casein polymorphism and diets, influencing the milk casein composition and yield.