We report a new biocompatible fiber-optic pH sensor based on optical fiber modal interferometer. Two biocompatible polyelectrolytes, i.e., sodium alginate (SA) and polyethylenimine (PEI), are alternatively deposited on the side surface of the thin-core fiber modal interferometer (TCFMI) via a layer-by-layer (LbL) electrostatic self-assembly (ESA) technique for pH sensing applications. With a measurement of water contact angles, it is revealed that the hydrophilicity of such a coating depends on the pH values. The refractive index (RI) of the coating is modulated through swelling/deswelling behavior when the pH of the solution is changed. The induced RI change is precisely determined by the TCFMI through a measurement of the destructive-interference wavelength. The thickness and morphology of the self-assembled sensing coating are characterized by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sensor exhibits a liner, fast, reversible and monotonic response in a large pH range with resolution of 0.0093 pH unit which makes the sensor very promising for biomedical and clinical pH measurement applications.