The metabolism of nitrate to nitrite by the oral microbiome has recently attracted considerable attention. Nitrate is abundant in the green and yellow vegetables comprising our daily diet. Nitrate is reduced to nitrite by the oral microbiome in the oral cavity, absorbed through the gastrointestinal tract after ingestion, and transferred to the bloodstream, where it is gradually reoxidized to nitrate, some of which is secreted back into the oral cavity as saliva (enterosalivary circulation). Consequently, the oral environment is constantly supplied with nitrate from food and saliva. Nitrite has antibacterial and vasodilatory effects, which may contribute to the suppression of oral or intestinal bacteria and decrease blood pressure through systemic vasodilation in the enterosalivary circulation. Thus, the nitrate metabolism of oral bacteria may play an important role in maintaining and improving both oral and systemic health. On the other hand, there has been concerns that nitrate may contribute to the production of carcinogenic substances such as nitrosamines. However, there is currently a mainstream view that this idea should be reconsidered. Given the growing number of reports showing the relationship between the oral microbiome and systemic health from the perspective of nitrate metabolism; accordingly, this review summarizes the latest findings on the mechanisms of nitrite production by the oral microbiome and its relationship with oral and systemic health.