The effect of fermented Rhus verniciflua stem bark (FRVSB) extract on the microbial count, enzyme activity, concentrations of free amino acids and organic acids, and physiochemical properties of doenjang (soybean paste) was evaluated during brine fermentation. The FRVSB extract increased the total free amino acid concentration by 1.3-3.1-fold on the 42 nd day of brine fermentation. After the filtration of brine, the following microbial counts were obtained in the doenjang: bacteria, 0.3 × 10 ⁸-12.0 × 10 ⁸ cfu/g; mold, 3.0 × 10 ⁴-21.0 × 10 ⁴ cfu/g; yeast, 1.0 × 10 ⁴-2.0 × 10 ⁴ cfu/g; Escherichia coli, not detected; and Bacillus cereus, 3.0 × 10 ²-25.0 × 10 ² cfu/g. The FRVSB extract addition enhanced the protein and starch degrading activity by 13.8–26.0% and 16.1–35.1%, respectively. The extract increased the total free amino acid content by 1.4-3.0-fold. Lactic acid, acetic acid, and pyroglutamic acid were the predominant organic acids in doenjang. Moreover, the proximate composition, pH, moisture, ash, salt, and amino nitrogen content were increased.