Here, we report a grapheme-polyaniline (Gr-PANi) composite modified glassy carbon electrode (GCE) as an amperometric pH sensor. Gr-PANi composite was synthesized by electro-deposition of PANi on the surface of Gr modified GCE using the cyclic voltammetry technique. XRD and Raman analysis confirmed high crystallinity of Gr-PANi composite. FESEM analysis revealed that Gr flakes are surrounded by PANi film because of the heterogeneous nucleation and growth of PANi during electro-polymerization. In acidic region, current of the composite-based pH sensor increased with decreasing pH values of the solution, whereas in alkaline region, current increased with the increasing pH values of the solution. The as-fabricated pH sensor exhibits shorter response time with an excellent sensitivity of - 50.14~\mu \text{A} pH-1 cm ^{-2} in the range of pH 1-5 and 139.2~\mu \text{A} pH-1 cm ^{-2} in the range of pH 7-11. The composite-based sensor showed enhanced sensitivity compared with pure Gr-based pH sensor because unlike pristine Gr-based pH sensor, it utilizes both charge-storage mechanisms in EDLCs and pseudo-capacitors, thereby resulting in excellent synergy between Gr and PANi. Reproducibility of the pH sensor was also examined between three different electrodes in the solution of pH 5. Insignificant variation in the current response suggested excellent reproducibility of pH sensor. To the best of our knowledge, no other studies have been reported on the Gr-PANi composite-based pH sensor over wide pH range. This Gr-PANi-based pH sensor offers a simple, low-cost, promising, real-time amperometric pH sensor in chemistry, clinical diagnostics, bio-sensing, and environmental monitoring applications. © 2001-2012 IEEE.