Rapid and inexpensive monitoring the real-time status of food products using pH sensors is critical for food quality and safety to determine if pathogens are present and growing. A promising material for pH sensors is ruthenium dioxide (RuO2) due to its chemical stability and excellent performance including: high sensitivity, low drift and hysteresis, and good selectivity. Furthermore, graphene oxide (GO) provides an electrode with large surface area, and good electrical properties. In this work, the in situ sol-gel deposition of RuO2 nanoparticles on the surface of GO as a facile, cost-effective, and environmentally friendly approach is used for the fabrication of a flexible pH sensor. The as-synthesized GO-RuO2 nanocomposites with a low volume were applied on the surface of screen printed carbon paste. The obtained GO-RuO2 nanocomposite pH sensor achieved high pH sensitivity (55.3 mV pH−1) in the pH range of 4–10, up to 4 times higher than the unmodified carbon electrode. The increased sensitivity of the modified electrode could be attributed to the uniform anchoring of small, crystallized RuO2 nanoparticles on the surface of GO sheets, resulting in synergistic effects between them. It also shows low drift (0.36 mV h−1) and low hysteretic width (0.8 mV). Considering the novel method of deposition and also sensing material with the cost-effective green synthesis approach, as well as excellent pH sensing properties, GO-RuO2 can be considered as a promising material for production of high-performance electrochemical pH sensors for food quality monitoring.