Mastitis is a common inflammatory disease that harms mammary gland health. Its development is closely linked to dysregulated inflammatory signaling. Eicosapentaenoic acid (EPA), an omega-3 polyunsaturated fatty acid, has potential anti-inflammatory effects. However, its molecular mechanism in mastitis prevention remains unclear. In this study, we used both in vivo and in vitro models to evaluate how EPA pretreatment regulates mastitis-related inflammatory signaling. Transcriptome analysis showed that differentially expressed genes after EPA treatment were mainly enriched in the peroxisome proliferator-activated receptor (PPAR) signaling pathway. In an LPS-induced mastitis model, EPA restored the LPS-reduced PPARγ protein level and suppressed NF-κB p65 activation, consistent with reduced nuclear translocation of p65. Similar effects were observed in mammary epithelial cells, where EPA inhibited NF-κB activation at 50 and 100 μM. Functional experiments further showed that a PPARγ agonist mimicked the inhibitory effect of EPA on p65, whereas PPARγ antagonist partially abrogated EPA-mediated inhibition of p65. Collectively, these data indicate that EPA attenuates mastitis-associated inflammation at least in part through the PPARγ–NF-κB axis.