Activated macrophages are classified into two different forms: classically activated (M1) or alternatively activated (M2) macrophages. The presence of M1/M2 phenotypic polarization has also been suggested for microglia. Here, we report that the secreted protein lipocalin 2 (LCN2) amplifies M1 polarization of activated microglia. LCN2 protein (EC50 1 μg/ml), but not glutathione S‐transferase used as a control, increased the M1‐related gene expression in cultured mouse microglial cells after 8–24 h. LCN2 was secreted from M1‐polarized, but not M2‐polarized, microglia. LCN2 inhibited phosphorylation of STAT6 in IL‐4‐stimulated microglia, suggesting LCN2 suppression of the canonical M2 signaling. In the lipopolysaccharide (LPS)‐induced mouse neuroinflammation model, the expression of LCN2 was notably increased in microglia. Primary microglial cultures derived from LCN2‐deficient mice showed a suppressed M1 response and enhanced M2 response. Mice lacking LCN2 showed a markedly reduced M1‐related gene expression in microglia after LPS injection, which was consistent with the results of histological analysis. Neuroinflammation‐associated impairment in motor behavior and cognitive function was also attenuated in the LCN2‐deficient mice, as determined by the rotarod performance test, fatigue test, open‐field test, and object recognition task. These findings suggest that LCN2 is an M1‐amplifier in brain microglial cells.—Jang, E., Lee, S., Kim, J.‐H., Kim, J.‐H., Seo, J.‐W., Lee, W.‐H., Mori, K., Nakao, K., Suk, K. Secreted protein lipocalin‐2 promotes microglial M1 polarization. FASEB J. 27, 1176–1190 (2013). www.fasebj.org