Porcine reproductive and respiratory syndrome virus (PRRSV) imposes substantial economic losses on global swine production. While modified live vaccines remain the primary prevention tool, their efficacy is compromised by the genetic variability of PRRSV. This study developed a broadly neutralizing monoclonal antibody (mAb) that targets a conserved viral epitope as an alternative therapeutic strategy. Using hybridoma technology, BALB/c mice with purified PRRSV particles and screened mAb-5F2 is immunized, which demonstrated cross-lineage neutralization against PRRSV-2 (lineages 1, 5, and 8) and PRRSV-1 strains. Epitope mapping revealed two residues, Q48 and I50, of GP4, which are key motifs for interaction with mAb-5F2. Mechanistic studies revealed that the mAb-5F2 sterically hinders the GP4-CD163 interaction, reducing viral entry efficiency by 78-85% at a concentration of 50 µg mL-1. To enhance clinical applicability, a porcine antibody (5F2-pFc) is engineered. In vivo challenge trials revealed that 5F2-pFc treatment completely prevented mortality in PRRSV-infected piglets (100% survival vs 40% in controls) while significantly reducing pulmonary viral loads (2-log decrease) and histopathological lesions. This work identified a novel conserved neutralizing epitope on PRRSV GP4 and established a porcineized antibody platform that combines broad-spectrum neutralization with clinical practicality, offering a promising strategy to overcome current vaccine limitations in PRRSV management.