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Douchi is a traditional Chinese fermented soybean product, in which acetoin is a key flavor substance. Here, the α-acetolactate decarboxylase gene <i>aldC</i> was cloned from <i>Lactiplantibacillus</i> (<i>L.</i>) <i>plantarum</i> and overexpressed in <i>Lactococcus</i> (<i>L.</i>) <i>lactis</i> NZ9000 by nisin induction. The ALDC crude enzyme solution produced an enzyme activity of 35.16 mU. Next, whole cells of the recombinant strain NZ9000/pNZ8048-<i>aldC</i> were employed as the catalyst to produce acetoin in GM17 medium. An optimization experiment showed that an initial OD₆₀₀ of 0.6, initial pH of 7.5, nisin concentration of 20 ng/mL, induction temperature of 37 °C and static induction for 8 h were the optimal induction conditions, generating the maximum acetoin production (106.93 mg/L). Finally, after incubation under the optimal induction conditions, NZ9000/pNZ8048-<i>aldC</i> was used for whole-cell biocatalytic acetoin production, using soybean as the substrate. The maximum acetoin yield was 79.43 mg/L. To our knowledge, this is the first study in which the <i>aldC</i> gene is overexpressed in <i>L. lactis</i> and whole cells of the recombinant <i>L. lactis</i> are used as a biocatalyst to produce acetoin in soybean. Thus, our study provides a theoretical basis for the preparation of fermented foods containing high levels of acetoin and the biosynthesis of acetoin in food materials.