A novel confocal optical system to study mixing time in small-scale bioreactors is presented. The system is designed to monitor fluorescence upon tracer addition from a localized confocal volume of 0.21 mL within a glass vessel. The key elements of the fluorescence-based confocal system are a pinhole, a lens, an APD (Avalanche photodiode) detector, and light filters. The optical technique was validated by comparison with a pH-based technique. Finally, the optical sensor was tested and a real cultivation media (i.e., spent mammalian cell media) was used to measure mixing time in a 12.5-mL stirred transparent vessel. High accuracy, easy results interpretation, and low costs are the three most attractive characteristics of the sensor. Because of its noninvasive nature and versatility, the results suggest that the confocal system is a promising tool to perform mixing time studies in stirred vessels.