A strain of Bacillus bacteria, which was able to increase the molecular weight (M) of black liquor (BL) lignin through polymerization, enabling the subsequent industrial use of lignin, was isolated and characterized. This study is believed to be the first time that actual bacteria cells, rather than pure laccase, have been used to polymerize BL lignin. Black liquor is a toxic waste product from the pulp and paper industry that contains lignin. However, the M of lignin is too low for commercial use. The bacteria performed two processes. First, the bacteria produced laccase, which degraded lignin into low M aromatic compounds (LMWACs). Second, the laccase transformed the LMWACs into quinone intermediates, which polymerized and became high M lignin. Five bacterial strains were isolated from a pulp mill, and the best strain was selected. The optimum growing conditions and BL concentration were determined. The optimum growth conditions when using pure lignin were 1 g/L lignin, 5 g/L urea, and 35 °C. When using BL instead of pure lignin, the optimum concentration was 2% BL (v/v). This information could help develop effective industrial utilization of BL lignin.