Elevating ambient temperatures driven by climate change are increasingly confronting animal productivity and health, particularly in heat-sensitive species like rabbits. Therefore, nutritional interventions are gaining attention for their role in enhancing heat tolerance. Brown seaweed Dictyota dichotoma (DD) extract is a rich source of bioactive compounds with potential health-promoting effects. Thus, this study assessed the protective influence of DD extract supplementation against natural heat stress in growing rabbits. A total of 120 rabbits were randomly assigned to four dietary treatments: a control diet (0 mg DD/kg) or diets enriched with 100, 200, or 400 mg DD extract /kg. The results showed that supplementation with DD extract significantly improved growth indicators. This improvement was seen in final body weight (P < 0.05) and feed intake (P < 0.001) in a quadratic effect, with the 400 mg/kg group showing the greatest gain (P < 0.001). Meanwhile, carcass quality and hematological parameters were significantly improved in a linear manner across all DD-supplemented groups (P < 0.001). The blood biochemical profiles showed decreased levels of creatinine, urea, ALT, AST, GGT, LDH, and sodium, indicating improved organ function (P < 0.05). The levels of thyroid hormones (T3 and T4) showed a quadratic increase, especially in the 200 mg/kg group (P < 0.001). Immune function was stimulated, as evidenced by increases in IgG, nitric oxide, lysozyme, and C3 concentrations (P < 0.001), while antioxidant capacity also improved through enhanced catalase, GPx, and TAC activities in a linear manner (P < 0.001). Concurrently, markers of oxidative stress (MDA, protein carbonyls, and 8-OHdG) were significantly reduced (P < 0.01). Moreover, acute-phase proteins such as Amyloid A and Haptoglobin were decreased (P < 0.001), while IL-10 levels rose (P < 0.001), supporting the extract’s anti-inflammatory potential. Finally, histological and immunohistochemical analyses confirmed that DD extract reduced tissue damage via regulating the HSP70 and MMP9 singling. In summary, dietary DD extract supplementation at a dose of 200–400 mg/kg effectively alleviated heat-induced physiological and immunological disruptions. This makes it a promising natural additive to enhance resilience and performance in heat-challenged rabbits.