<p>The determination of bacterial concentration in water is a critical task in multiple fields, including monitoring of water courses in the environment, industrial effluents, industrial processes and network water monitoring. This study presents a novel approach that uses a biospeckle laser to estimate real-time bacteria concentration in water. The bacteria act as optical scatterers, reflecting a coherent light from a laser and generating a speckle image that evolves over time in response to sample activity. Through dedicated algorithms to process the acquired videos of the sample, the detected activity is quantified as a descriptive value which is then correlated with the known bacterial concentration in a reference table. The proposed method is sensitive to low bacterial density and its performance is demonstrated for various concentrations of bacteria in sterile physiological solution. Since the ultimate goal is the implementation of a field device, testing with an inexpensive solid-state laser was included. This method can serve as a powerful tool for fast, simple, and cost-effective on-site water monitoring.</p>