Dendropanax morbifera (DM) extract was co-fermented by Bacillus subtilis HA and Lactobacillus plantarum EJ2014 for producing poly-γ-glutamic acid (γ-PGA) and γ-aminobutyric acid (GABA). The first fermentation with B. subtilis HA resulted in a viscous broth with pH 7.85, 0.01% acidity, 0.72 mg/g peptides, 17.39 U/g protease activity, 8.44 log CFU/mL B. subtilis, 4.96 Paos_n consistency index, and 45.84 mg/mL mucilage. For the second fermentation with L. plantarum, the broth was enriched with skim milk. The co-fermented broth indicated 0.5% acidity and pH 5.1. The viable cell counts of B. subtilis decreased, whereas those of L. plantarum increased, as did the peptide content. The MSG precursor was effectively converted to GABA, which showed a concentration of 12.80 mg/mL. Therefore, the co-fermented DM extract was enriched with γ-PGA, GABA, peptides, and probiotics by the serial co-fermentation, and could be used as a potential ingredient for developing functional foods.