A fluorescent sensor based on a diketopyrrolopyrrole (DPP) derivative for protein detection was designed and tested in urine samples. The DPP derivative emits visible light as a solid, making this molecule particularly appealing as a recognition molecule for a sensing optical fibre. In the presence of proteins, the sensor displayed a fluorescence enhancement. The sensor linear dynamic range for protein ranges from 10 mg L-1 to 100 mg L-1. Detection limit (LOD) of this fluorescent optrode was better than the one of the classical spectrophotometric Biuret method and similar to the LOD of the spectrophotometric Ponceau-S/TCA method. Economy of reagents and time is a clear advantage of the analytical method based on the optrode we are now proposing, regarding the Ponceau-S/TCA, thanks to the reversibility of recognition layer, and to the needleless of treating the standards with trichloroacetic acid, and consequently of centrifuging them. Besides, the most common interferences in protein determination by classical spectrophotometric methods, like sodium dodecyl sulfate, acetone, and glucose, do not affect the optrode response, neither does fluoride ion. The new fluorescent optrode was tested in the determination of total proteins in urine from healthy individuals (low concentrations). Results were not statistically different from the ones obtained with the Ponceau-S/TCA spectrophotometric method.