The economic viability of vanadium flow batteries (VFBs) depends on maintaining optimal power output and energy capacity throughout their lifespan. A key factor in achieving this is the precise monitoring of the electrolyte-specific state of charge (SoC). This work presents a novel open-circuit-potential (OCP) cell designed for real-time, cost-effective, and reliable SoC monitoring in VFBs. The OCP-cell measures the OCP of the anolyte and catholyte relative to a vanadium-based reference electrolyte, ensuring compatibility and minimizing contamination risks. The relationship between cell potential and electrolyte composition is governed by the Nernst equation, with additional considerations for Donnan potential effects due to proton concentration gradients across the membrane. Electrochemical impedance spectroscopy (EIS) was employed to analyze the OCP-cell’s internal resistances, revealing that membrane resistance is the dominant factor affecting high-frequency impedance (ohmic resistance), while charge transfer resistance is elevated due to the small electrode surface area. The cell’s low DC resistance ensures accurate OCP measurements without significant voltage drop. Experimental cycling tests using 1.6 M vanadium electrolyte demonstrated the OCP-cell’s accuracy, with deviations from reference OCV-cell measurements remaining within a narrow range (−0.3 ± 0.4 mV). A slight drift in the determined electrolyte-specific SoC was observed, presumably due to vanadium ion crossover in the main cell. Future research will focus on improving the correlation between electrolyte-specific SoC and measured OCP values, ultimately enhancing the efficiency, longevity, and cost-effectiveness of VFB technology.