Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by deficits in social communication and interaction, and repetitive behaviours. Numerous studies have associated ASD with immune dysregulation and inflammation, with neuroinflammatory processes reported in ASD individuals and mouse models. Altered immune cell profiles and cytokine levels have been observed in the peripheral blood (PB), supporting systemic immune dysfunction. Recently we showed that the administration of antioxidant molecule N-acetylcysteine (NAC) reduced oxidative stress and inflammation and counteracted behavioural deficits in two mouse models of ASD, providing a rationale for exploring other redox-active compounds. Here, we investigated the effects of astaxanthin (AST), potent antioxidant and anti-inflammatory molecule, in the Shank3b model ( Shank3b ^-/- mice). AST treatment significantly improved core ASD-like behaviours, including social interaction deficits, motor incoordination, and repetitive grooming. In the cerebellum, AST reduced pro-inflammatory cytokines and counteracted microglial hyperactivation. In peripheral immune compartments, AST modulated cytokine expression. Pro-inflammatory markers were downregulated in Shank3b ^-/- mice in the bone marrow and spleen while they were elevated in Shank3b controls, suggesting immune rebalancing ( i.e. adaptive modulation suppressing harmful inflammation while supporting protective immunity). As a limitation, oxidative stress assays were not performed here. Receiver operating characteristic (ROC) analysis suggests that TNF and IFNγ expression in peripheral immune cells may be promising biomarkers of treatment response. Notably, unlike NAC, AST did not induce pro-inflammatory effects in Shank3b ^+/+ animals. These findings show that AST administration may counteract behavioural deficits and immune dysfunction in Shank3b ^-/- mice, therefore suggesting its potential as a safe immunomodulatory therapy for ASD.