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Simulated spaceflight perturbs multiple organ systems, yet the integrated impact of spaceflight-relevant stressors on the immune-gut-brain axis remains poorly defined. We used a ground-based model combining hindlimb unloading (HU) with low-dose ionizing radiation (IR; 50 or 100cGy) to quantify neuropathology, peripheral immune phenotypes, intestinal barrier integrity, and behavioral performance in male and female C57BL/6 mice. HU and/or IR induced region-selective neurodegenerative changes consistent with axonal injury across the cortex and major white-matter tracts. In the somatosensory cortex, MAP-2+ neurons were reduced and SMI-312-labeled axonal injury increased, lowering the intact-to-dystrophic axonal area ratio. Long-range fiber pathways (corpus callosum, cingulate gyrus, external capsule) showed robust axonal damage accompanied by gliosis, with elevated Iba-1+ microglia and GFAP+ astrocytes most prominent after HU+IR (100cGy). Peripheral immunophenotyping revealed a sustained, sex-dependent innate inflammatory bias, with expanded CD11b+ myeloid cells and increased TNF-α+ myeloid activation after IR and IR+HU, alongside maladaptive T-cell polarization despite largely unchanged total CD8+ and CD4+ frequencies. In parallel, the gut exhibited architectural remodeling and barrier failure, including altered mucin profiles, reduced ZO-1 tight-junction labeling, and increased CD45+ leukocyte infiltration across the jejunum, ileum, and colon. Behavioral assays demonstrated sex-dependent deficits spanning affective, motor, and cognitive domains, including increased anxiety- and depressive-like behaviors, impaired rotarod performance, reduced recognition memory, and less efficient spatial strategies. Overall, these findings identify a sex-dependent immune-gut-brain vulnerability in which combined HU and low-dose IR drive gut barrier breakdown and immune imbalance that coincide with neuroinflammatory axonopathy and measurable neurobehavioral dysfunction.