Background <p>Previous studies on intraventricular hemorrhage (IVH), a common and severe complication of preterm birth, and subsequent post-hemorrhagic hydrocephalus (PHH), have predominantly concentrated on the secretory function of the choroid plexus (ChP), with considerably less emphasis on its barrier function. We hypothesized that PHH is associated with immune-related alterations in the junction biology of ChP.</p> Methods <p>We examined differences in tight junctions and macrophages using a neonatal mouse model of PHH (<i>n</i> = 40) and in vitro ChP explants (<i>n</i> = 22), as well as human post-mortem samples (<i>n</i> = 6). To test our hypothesis, we employed histology, immunofluorescence, magnetic resonance imaging, spectral flow cytometry, fluorescence activated cell sorting, and transmission electron microscopy.</p> Results <p>In the mouse model, we observed a significant increase (<i>p</i> = 0.0025) in ventricular volume in the PHH group compared to sham controls. PHH was associated with a significant increase (<i>p</i> = 0.0177) in the number of macrophages in the ChP. These macrophages displayed an activated phenotype, characterized by numerous phagosomes and lysosomes observed by transmission electron microscopy, and quantified by CD68 immunostaining (<i>p</i> = 0.0003). Further, we identified significant decreases (<i>p</i> = 0.0048 and <i>p</i> = 0.033, respectively) in tight junction proteins ZO-1 and claudin-1 in the epithelial cells of the ChP in PHH. In vitro co-cultures of peripheral CD11b⁺ Ly6G⁻ Ly6C⁺ cells (precursors of ChP macrophages) and lysed blood demonstrated significant disruption (<i>p</i> = 0.0046) of tight junctions in ChP. This disruption in ZO-1 was not observed when ChP were cultured only with lysed blood and without CD11b⁺ Ly6G⁻ Ly6C⁺ cells. The findings of tight junction disruption in the ChP epithelial cells and the significant increase (<i>p</i>≤0.05) in macrophages were confirmed in preterm human post-mortem ChP samples.</p> Conclusions <p>These results suggest that IVH/PHH is associated with an increased in activated macrophages in the ChP and impaired tight junctions in the ChP epithelium. This research opens avenues for exploring novel immunomodulatory treatments aimed at preventing the pathogenesis and neurodevelopment impairments common in PHH.</p>