The development, implementation, and demonstration of in-fiber Bragg grating (FBG) sensors for a simultaneous measurement of temperature and relative humidity (RH) in an operating polymer electrolyte membrane fuel cell (PEMFC) are presented. Etched fiber, polymer-coated FBG sensors with fast response and high sensitivity are installed in situ in a minimally invasive manner in the cathode unipolar plate of a single PEMFC with serpentine flow fields. The performance of the fuel cell under transient operating conditions is monitored. Step increases in current induce significantly larger increases in RH near the outlet than near the inlet of the cell, and associated transients within the fuel cell are found on a time scale approaching the sensor response time (~1 Hz). The improved response of the technique, together with the significantly improved temperature and RH resolution, provides useful information on the dynamics of heat and water management. The technique is well suited for distributed measurements, and its relatively low cost and nonintrusive character make it a good candidate for practical multipoint monitoring of complete stacks.