Hydrogen production via polymer electrolyte membrane (PEM) water electrolysis has attracted considerable attention as a promising technology to store renewable electricity and combat global warming. Although PEM water electrolyzers can produce high-purity hydrogen at high current densities, the use of low-purity water leads to device degradation because metal ions from the water are deposited on the membrane, thereby increasing its resistance and operating voltage. In this study, an in-situ recovery method was developed, in which degraded PEM water electrolysis cells were chemically treated without disassembly. Cells after degradation were subjected to a 24-h chemical treatment with either a strong acid (1.0 mol/L nitric acid) or a weak acid (12.9 and 1.0 mol/L phosphoric acid), followed by the supply of ultrapure water for 72 h. Recovery was evaluated using cell voltage measurements, while scanning electron microscopy (SEM)-dispersive X-ray spectroscopy (EDX) and inductively coupled plasma (ICP) analyses were performed to investigate membrane morphology, elemental distributions, and metal ion removal. Among the tested acids, 12.9 mol/L phosphoric acid showed the highest voltage recovery performance, achieving a 90% recovery ratio immediately after treatment (0 h). Moreover, a comparison of the voltage recovery ratios at 1 h post-immersion suggests that higher hydrogen ion concentrations are more effective for the recovery of degraded PEMs. These findings demonstrate that in-situ acid treatment can restore the performance of contaminated PEM water electrolyzers without disassembly and may provide a practical approach for extending cell lifetime and reducing maintenance requirements.