We present measurements of relative humidity using a novel FBG sensor design embedded within the active area of a single cell. The fabrication, calibration, and installation methods for the Bragg grating based sensors are presented. The sensors are located at two distinct channel and land locations on both anode and cathode, and the measurements are compared with data from capacitive sensors located at the edges of the flow field. Experiments conducted under dynamic conditions show significant differences between land and channel locations, particularly on the cathode side, and suggest that it may not be appropriate to use RH measurements in the channel to infer the membrane hydration level on the cathode side during operation. The response time of the FBG sensor also allows resolution of RH transients associated with electro-osmotic drag and back diffusion. The study shows that these FBG sensors can be integrated in a PEM fuel cell with minimal impact on performance, and allow spatially and time resolved measurements in regions inaccessible by other techniques. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.