A flexible, screen-printed potentiometric biosensor was developed for the continuous, noninvasive monitoring of sodium concentration in human sweat. The sensor employed a poly(vinyl chloride) membrane doped with sodium ionophore X and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, with multiwalled carbon nanotubes (MWCNTs, 0–1 wt%) incorporated to enhance ion-to-electron transduction. Structural characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron Spectroscopy (XPS) confirmed homogeneous membrane formation and effective nanotube integration. Electrochemical analysis revealed a progressive improvement in performance with MWCNT loading: the sensitivity increased from 45.5 ± 1.2 to 58.2 ± 0.7 mV dec−1, the detection limit decreased to 9.1 × 10−5 M, and the potential drift was minimized to 0.1 mV h−1. The optimized device responded within 10 s and maintained stable slopes after 5000 bending and 2000 stretching cycles, demonstrating strong mechanical durability. Selectivity coefficients (log KNa,jpot ≈ −2.5 to −3.4) confirmed negligible interference from K+, NH4+, Ca2+, and Mg2+. During on-body trials involving 10 subjects performing 60 min of cycling at 27 ℃, the wearable sensor accurately captured dynamic sweat Na+ profiles ranging from 45 to 70 mM. Sensor readings correlated excellently with ion-chromatography reference values (R2 = 0.985, mean bias –1.2 mM). These results validate the device as a reliable real-time platform for individualized fluid-electrolyte assessment. The integration of nanocomposite solid-contact membranes with flexible printed electronics enables the precise quantification of sweat electrolytes, supporting data-driven, personalized hydration management during exercise and heat exposure.