γ-Aminobutyric acid (GABA) is a potentially bioactive ingredient with health-promoting properties that is added to functional foods. Streptococcus thermophilus was selected to produce naturally GABA-enriched fermented milk. This strain can yield a GABA concentration of 2.8 g/L after a 48-h fermentation. In the presence of 1 g/L food-grade casein hydrolysate as a nitrogen source, S. thermophilus yielded GABA concentrations as high as 5.4 g/L or even 8.3 g/L when cocultured with Lactobacillus rhamnosus. In other words, both of these added conditions promoted GABA enrichment. The GABA dose achieved with fermented milk was comparable to the doses of commercially available GABA supplements. Additionally, the in situ use of S. thermophilus to produce GABA-enriched fermented milk was cost effective. The complete genomic sequence of S. thermophilus GABA has been published and will be highly useful to other researchers studying the regulation of genes related to GABA accumulation. In conclusion, the S. thermophilus GABA-producing strain reported herein represents a natural method for the production of fermented milk containing high GABA concentrations.