Periodontitis is a localized infectious disease caused by periodontopathic bacteria, such as Porphyromonas gingivalis. Recently, it has been suggested that bacterial infections may contribute to the onset and the progression of Alzheimer’s disease (AD). However, we do not have any evidence about a causative relationship between periodontitis and AD. In this study, we investigated by using a transgenic mouse model of AD whether periodontitis evoked by P. gingivalis modulates the pathological features of AD. Cognitive function was significantly impaired in periodontitis-induced APP-Tg mice, compared to that in control APP-Tg mice. Levels of Amiloid β (Aβ) deposition, Aβ40, and Aβ42 in both the hippocampus and cortex were higher in inoculated APP-Tg mice than in control APP-Tg mice. Furthermore, levels of IL-1β and TNF-α in the brain were higher in inoculated mice than in control mice. The levels of LPS were increased in the serum and brain of P. gingivalis-inoculated mice. P. gingivalis LPS-induced production of Aβ40 and Aβ42 in neural cell cultures and strongly enhanced TNF-α and IL-1β production in a culture of microglial cells primed with Aβ. Periodontitis evoked by P. gingivalis may exacerbate brain Aβ deposition, leading to enhanced cognitive impairments, by a mechanism that involves triggering brain inflammation.