In this study, a cost-effective optical sensor was developed to quantify a blood loss during transurethral surgery. The device is based on the principles of optical spectroscopy and consists of an ultraviolet light emitting diode (LED) with an emission maximum of 405 nm as a light source, and a simple photoresistor for detecting the transmitted light. Arduino UNO board provides continuous voltage measurements of the photodiode, and sends these values to a computer for processing. This optical sensor was installed into the drain tube connected to the resectoscope where saline with a possible presence of blood was channeled during a transurethral resection. To calculate the blood loss, the sensor signal intensity over time was plotted to generate an “area under the curve” measurement. The performance of the device was evaluated in the laboratory by assaying different concentrations of human blood mixed with a saline solution. Measurements were made using both static and flowing samples. Linear regression models for both modes have a high correlation coefficient between the area under the curve and the blood content (R² = 0.99 and 0.93, for static and dynamic measurements, respectively). The optical sensor received practical testing during a transurethral resection of a bladder tumor. The results indicate that the developed optical sensor can be successfully applied in medical practice.