<p>The intensification of aquaculture, along with global bans on antibiotic growth promoters, has created an urgent need for sustainable immunomodulatory strategies to enhance fish health and welfare. Hence, this study evaluated, for the first time, the immunomodulatory effects of <i>Lagenaria siceraria</i> seed (LS) supplementation in European seabass <i>(Dicentrarchus labrax)</i> by assessing its impact on gut integrity, blood health markers, growth attributes, digestive enzyme activity, antioxidant status, immune-related gene expression, and resistance to <i>Vibrio alginolyticus</i> infection. A total of 204 fish (19.07 ± 0.06&#xa0;g) were randomly distributed into four dietary treatment groups including a control basal diet (LS0) and three experimental diets supplemented with LS at concentrations of 5 (LS5), 10 (LS10), and 15&#xa0;g/kg (LS15). Fish in the LS groups exhibited higher growth indices (averaged daily gain (ADG), final body weight, fish biomass, specific growth rate (SGR), and feed conversion ratio (FCR)), and digestive enzymes activity (lipase, amylase, trypsin, and chymotrypsin) compared to the LS0 group (<i>P</i> &lt; 0.05), with the best results observed in the LS10 and LS15 groups. Dietary LS15 supplementation significantly optimized body composition, leading to maximum crude protein and dry matter levels while minimizing crude lipid and moisture content compared to the LS5 and LS0 treatments (<i>P</i> &lt; 0.05). LS inclusion improved antioxidant status including superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPx) and lysosome activity, while significantly reducing malondialdehyde (MDA) concentrations compared to the free-LS diet (<i>P</i> &lt; 0.05). The LS15 diet notably enhanced blood health and protein profiles, while decreasing hepatic enzyme markers compared to the LS5 and control groups (<i>P</i> &lt; 0.05); the LS10 group showed an intermediate response. Dietary LS supplementation upregulated the mRNA expression of complement component 3 (C3), SOD, and GPx genes significantly, while downregulating the expression of interleukin-1β (IL-1β) gene, indicating potent antioxidant and anti-inflammatory effects (<i>P</i> &lt; 0.05). Intestinal morphometric analysis revealed that LS-supplemented groups exhibited superior gut integrity, with significant improvements in villus length, width, and overall absorptive capacity compared to the LS0 group. A decreasing trend in cumulative mortality following <i>V. alginolyticus</i> challenge was observed in fish fed LS-supplemented diets, particularly in the LS15 group, although the effect did not reach statistical significance (<i>P</i> &gt; 0.05). Overall, these findings demonstrate that <i>Lagenaria siceraria</i> seed supplementation significantly enhances growth performance, disease resistance, metabolic health, and intestinal integrity in European seabass due to its potent immunomodulatory and antioxidant properties, especially at the 15&#xa0;g/kg inclusion level. The results suggest that <i>L. siceraria</i> seed meal may be a promising functional feed additive for seabass culture.</p>