Posidonia oceanica (L.) Delile, a Mediterranean seagrass, is rich in bioactive compounds with anti-inflammatory potential. While marine-derived molecules are increasingly studied, their direct effects on blood-brain barrier (BBB) integrity under inflammatory conditions remain largely unexplored. This study evaluated the ability of aqueous extracts from its green leaves (GLEs) and rhizomes (REs) to protect the BBB using a human in vitro model consisting of brain-like endothelial cells co-cultured with brain pericytes. The model was exposed to TNFα, with or without GLEs or REs. We assessed NO production, endothelial permeability, expression of IL-6, NLRP3, ICAM-1, VCAM-1, CLAUDIN-5, and VE-CADHERIN, and the localization of junctional proteins. TNFα increased NO and IL-6 release, upregulated ICAM-1, VCAM-1, and NLRP3, and impaired BBB integrity by altering junctional protein levels and distribution. Co-treatment with GLEs or REs reduced the production of NO, the expression of NLRP3 and adhesion molecules and restored tight and adherens junction integrity. IL-6 levels remained unaffected. These findings suggest that P. oceanica's extracts may help preserve BBB function and mitigate inflammation-induced damage. While further studies are needed to assess their bioavailability and in vivo efficacy, these natural compounds represent promising candidates for developing preventive strategies against neuroinflammatory disorders.