Water starvation could be one of the reasons for proton exchange membrane (PEM) water electrolyzer degradation. In this paper, the water starvation phenomena of a unit cell in a PEM electrolyzer stack are investigated. The voltage, current density and temperature distribution are investigated in situ with a segmented electrolyzer. The results show that the voltage of the middle and outlet regions is higher than the inlet voltage, which illustrates that water starvation could occur simultaneously in different regions of the electrolyzer. The water stoichiometries have an important effect on the voltage distribution, current density distribution and temperature distribution at 0.5 A cm⁻² and 60 °C. The electrochemical impedance spectra of different segments show that the cell resistance and charge transfer resistance gradually increase along the water flow direction when the water stoichiometry is 3. According to the flow regime map, the critical water stoichiometry for electrolysis is further discussed.