Poly(ethylene terephthalate) (PET) fibers are widely used in various fields owing to their good physical properties and chemical resistance, but PET hydrophobicity heavily compromises its use in many applications. Wet-chemical treatments are often required to induce hydrophilicity, but these can damage the polymer matrix and produce large volumes of liquid wastes. Atmospheric-pressure glow-discharge plasma (APGDP) is an alternative and ecofriendly method to obtain similar or better results with polymeric materials, compared to wet-chemical treatments. The hydrophilic behavior of PET samples is investigated after a roll-to-roll APGDP treatment by varying the plasma gas mixture, gas fluxes, and electrode temperatures. The reactive species formed in the plasma chamber are characterized by optical emission spectroscopy. The induced surface functionalization and roughness are characterized by contact-angle measurements, atomic force microscopy, and UV/Vis absorbance of a probe dye.