This study investigates the flexural behaviour of reinforced concrete beams strengthened with a hybrid reinforcement system comprising both steel and fibre-reinforced polymer (FRP) bars, with a specific focus on serviceability deflection performance. The incorporation of FRP reinforcement offers significant advantages in terms of corrosion resistance and reduced self-weight, making it an attractive complement to conventional steel reinforcement. However, the linear-elastic response of FRP up to failure, combined with its lower modulus of elasticity, limits its suitability as a complete replacement for steel. Hybrid reinforcement systems therefore present a promising compromise, combining the ductility of steel with the durability of FRP. An extensive database of experimental tests on hybrid reinforced beams, compiled from the literature, was analysed. Measured mid-span deflections were compared with predictions from established guidelines. The findings indicate that many existing models tend to underestimate deflections, which may have implications for both serviceability and long-term structural performance. To address these discrepancies, the analytical models were recalibrated by introducing correction factors derived through multiple error functions, including mean squared error (MSE), mean absolute error (MAE), mean absolute percentage error (MAPE), and the coefficient of determination (R²). Finally, a parametric analysis identified the most influential variables affecting deflection, including the stiffness ratio between steel and FRP reinforcement, the proportion of FRP reinforcement, and the shear span-to-depth ratio. The results provide valuable guidance for optimising hybrid reinforcement design and contribute towards the development of dedicated design provisions, addressing current gaps in structural codes and promoting the wider adoption of steel–FRP hybrid systems in civil engineering practice.