Soy molasses, a by-product from the processing of soy protein concentrate, is a low-cost feedstock for fermentation processes due to its high content of fermentable sugars. This work investigates the use of soy molasses for growing Bacillus species, aiming at their potential application as plant growth promoters. Firstly, six Bacillus strains were screened for their ability to grow in increasing concentrations of soy molasses in a microplate assay. Following this, shaken-flask assays for growth and γ-polyglutamic acid (γ-PGA) production by three Bacillus strains in medium E and soy molasses media with 28 and 56 g L⁻¹ of total reducing sugars (TRS) were carried out. An in vivo experiment evaluated the effect of the bacterial fermented broths on the germination and initial development of maize. Soy molasses supported the growth of Bacillus amyloliquefaciens, Bacillus subtilis, and Bacillus licheniformis in concentrations of 28 and 56 g L⁻¹ TRS, but it was inhibitory at 112 and 224 g L⁻¹ TRS. In soy molasses media, growth was not always associated with γ-PGA production, which was a maximum of 56 g L⁻¹ TRS for B. amyloliquefaciens and B. licheniformis. Fermented broths with B. subtilis and B. licheniformis in soy molasses media (56 and 28 g L⁻¹ TRS, respectively) applied to maize seeds resulted in the highest Vigor Indexes of the seedlings, which correlated negatively with the broth pH and were not impacted by the γ-PGA and indole acetic acid produced by the bacteria. The low-cost and easily available feedstock soy molasses constitutes a potential culture medium for the growth of plant growth-promoting bacteria.