Water is the foundation of all life, yet increasing human, industrial, and agricultural activities have placed immense pressure on its quality. Pollution of water resources poses serious risks not only to public health but also to the stability of ecosystems. For this reason, effective water treatment has become an urgent global priority. Among the many biological agents that contribute to cleaner water, Bacillus subtilis stands out as a particularly important microorganism. Widely used in biological wastewater treatment, this bacterium plays a central role in purifying water, breaking down organic matter, and removing excess nutrients such as nitrogen and phosphorus. Its significance lies in its natural ability to produce powerful enzymes—including proteases, lipases, and amylases—that degrade proteins, fats, starches, and other complex organic compounds. By harnessing these enzymatic processes, Bacillus subtilis offers a sustainable, cost-effective, and environmentally friendly alternative to purely chemical treatment methods. This study explores the physiology and ecological role of Bacillus subtilis in water treatment systems, drawing upon insights from scientific journals, books, and research articles. Using a descriptive approach, relevant materials were collected, reviewed, and synthesized to provide a clear overview of the bacterium’s importance. The findings highlight that Bacillus subtilis is highly effective in reducing key pollution indicators such as Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD), along with other hazardous contaminants. These results underline its potential as a cornerstone in modern biotechnological approaches to wastewater management. Based on this evidence, the study recommends greater focus on Bacillus subtilis-based biotechnologies as a natural, affordable, and reliable strategy for improving water quality and safeguarding both human and environmental health.