Exposure to lipopolysaccharide (LPS) causes cognitive impairment (CI). In the preliminary study, Lactobacillus gasseri NK109 suppressed LPS-induced expression of proinflammatory cytokines in macrophages. Therefore, the effect of NK109 on LPS-increased CI was investigated in mice. Intraperitoneal injection of LPS caused CI-like behaviors and neuroinflammation. However, orally administered NK109 reduced LPS-increased CI-like behaviors and hippocampal IL-1β and TNF-α expression, whereas LPS-decreased BDNF expression increased. NK109 also reduced LPS-increased colonic myeloperoxidase, IL-1β, and TNF-α expression. The efficacy of NK109 was increased by the combination of soybean embryo ethanol extract (SE). These findings suggest that NK109 with SE can improve CI by alleviating inflammation-mediated BDNF expression, thereby being beneficial for dementia therapy.