The intracellular pathogen Shigella flexneri replicates to high cell densities in colon epithelial cells and then spreads to neighbouring cells. This study found that formate (a metabolite of S. flexneri glycolysis and mixed acid fermentation within host cells) accumulation enhances virulence gene expression and intercellular spread. A mutant of pyruvate formate lyase (PFL; ΔpflB), which cannot convert pyruvate to acetyl-CoA and formate, produced smaller plaques in epithelial cell monolayers. The growth rate of this mutant was not affected, but its ability to spread cell to cell was. This phenotype was rescued by adding exogenous formate or by deleting the S. flexneri formate dehydrogenase gene fdnG, which increased formate accumulation. Formate was found to increase the expression of S. flexneri virulence genes icsA and ipaJ, which promote intercellular spread and inhibit the trafficking of the host innate immune effector STING, respectively, suggesting that formate is an intracellular signalling molecule that regulates virulence in response to S. flexneri metabolism and density.