Astrocytes are critical components of the neurovascular unit that support blood-brain barrier (BBB) function in brain microvascular endothelial cells (BMECs). Transformation of astrocytes to a reactive state in response to injury and disease can be protective or harmful to BBB function, but the underlying mechanisms for these effects remain mostly unclear. Using a human induced pluripotent stem cell (iPSC)-derived coculture model of BMEC-like cells and astrocytes, we found that tumor necrosis factor alpha (TNFα) transitions astrocytes to an inflammatory reactive state through activated STAT3 signaling, whereby the resultant astrocytes disrupt passive BBB function and induce vascular cell adhesion molecule 1 (VCAM-1) expression in the BMEC-like cells. These associations between inflammatory reactive astrocytes, STAT3 activation, and vascular VCAM-1 expression were corroborated in human postmortem tissue. Bioinformatic analyses coupled with CRISPR interference techniques in the iPSC model revealed that inflammatory reactive astrocytes transduce BBB disruption in part through SERPINA3 , which encodes alpha 1-antichymotrypsin (α1ACT), a secreted serine protease inhibitor associated with aging, neuroinflammation, and Alzheimer’s disease. In murine ex vivo cortical explant cultures, shRNA-mediated silencing of Serpina3n in astrocytes reduced vascular VCAM-1 expression after TNFα challenge. Further, direct treatment with recombinant Serpina3n in both ex vivo explant cultures and the brain in vivo (via intracerebroventricular injection into wild-type mice) was sufficient to induce vascular VCAM-1 expression and reduce tight junction integrity. Overall, our results define the TNFα-STAT3 signaling axis as a driver of an inflammatory reactive astrocyte subtype responsible for BBB dysfunction. Our results also identify α1ACT as an explicit mediator of BBB damage and suggest that inhibition of α1ACT expression or activity could represent a therapeutic avenue for reversing BBB deficits in aging and neurodegenerative disease.