Salt stress inhibits plant growth and development and plants activate kinase-dependent survival pathways in response to salt stress. However, the role of soybean mitogen-activated protein kinases (MAPKs) in salt stress response has yet to be characterized. In this study, we found that salt stress activates Glycine max MAP kinase 1 (GMK1), a soybean MAPK. The activity of GMK1 induced with increasing salt concentrations, up to 300 mM NaCl, after 5 min of the treatment and was regulated by post-translational modification. We found that mastoparan, a heteromeric G-protein activator, also activated GMK1, and that n-butanol, a phospholipase D inhibitor, and neomycin, a phospholipase C inhibitor, inhibited its activity. Moreover, GMK1 activity was reduced by suramin, a heteromeric G-protein inhibitor, and by two inhibitors of phosphatidic acid (PA) generation after 5 min of 300 mM NaCl treatment. Endogenous PA levels were highest 5 min after induction of salt stress, and exogenous PA directly activated GMK1. From these data, we propose that salt stress signaling is transduced from heteromeric G-protein to GMK1 via phospholipases in the early stages of the response to salt stress in soybean.