In higher plants, glutamate decarboxylase (GAD, EC 4.1.1.15) is known as a key enzyme in the synthesis of γ-aminobutyric acid (GABA), which is not only a valuable food component for animals, but is also a signaling molecule in plants. Although the completion of the soybean genome sequencing has made it possible for the identification of GAD gene families, the systematic analysis of GAD genes in soybean has not yet been reported. Based on the recently released soybean whole-genome sequences, in this study, we identified five GAD genes that contain conserved residues and calmodulin (CaM)-binding domain in the C-terminal region. This indicates that the putative soybean GADs (GmGADs) are Ca 2+ /CaM-binding proteins. An abundance of GmGAD4 and 5 transcripts have been observed in cotyledons, whereas GmGAD1 and 3 were expressed in hypocotyls and roots of 7-day-old soybean sprouts. In addition, we have determined a positive correlation between the expression of GmGAD genes and GABA accumulation during the germination process. Furthermore, the induction of GmGAD1, 4, and 5 transcripts following infection of hypocotyls with Phytophthora sojae suggests that GABA may play a role in the resistance mechanism. Taken together, our comparative genomic analysis of GAD genes and encoded proteins in soybean is the first step toward the functional dissection of this family, and we present information that will assist future studies on GABA metabolism and signaling research.