An in-line intrinsic Fabry-Perot refractive index (RI) sensor is demonstrated. The sensor consists of a short single-mode fiber (SMF) section with removed cladding that is fusion spliced between two lead fibers containing in-fiber mirrors. The measured medium surrounding the decladed fiber section affects the fundamental mode's effective index and, consequently, the optical path length of the Fabry-Perot interferometer (FPI). Sensors with different diameters of decladed regions were produced and experimentally evaluated within an RI range of between 1.33 and 1.444. The proposed sensor can be interrogated by standard spectrally resolved interrogators.