The optimal production of γ-aminobutylic acid (GABA) has been accomplished using the extract of Phellinus linteus fruiting body (PLF) and Lactobacillus plantarum KS2020. To optimize the production of GABA through lactic acid fermentation, the medium composition was optimized for the concentration of glucose, monosodium glutamate (MSG), and yeast extract. The solid content, pH, acidity of PLF extract was 1.37%, 5.85, and 0.03%, respectively. The content of the minerals Ca, Mg, K, Na, P, Fe, and Mn in the PLF extract were 3.65, 2.07, 1.95, 1.43, 0.88, 0.05, and 0.01 mg per 100 g, respectively. In case of single lactic acid fermentation with the PLF extract including 5% MSG as a precursor and 1.5% yeast extract, the pH and acidity of the fermented PLF extract were 5.96, and 0.25%, respectively, and the viable cell count was 8.56 log CFU/mL. After 9 days of stationary fermentation, MSG was converted to highly concentrated GABA. High performance liquid chromatography (HPLC) analysis revealed that, the initial content of GABA (21.18 mg/100 g) increased to 2,412.34 mg/100 g in the fermented PLF extract. In conclusion, the fermented PLF extract using L. plantarum KS2020 and MSG as a precursor can be used for the production of high-value functional food ingredients, such as GABA and probiotics.