To improve the adsorption capacity of polyethylene terephthalate (PET) fibers in antibiotic removal processes, the functionalization with sodium alginate can be a viable solution. In this study, PET fibers were functionalized with sodium alginate of different concentrations (0.1, 0.5, and 1.0%) and used for the retention of rifampicin (RIF) from aqueous media. Experimental results showed that capacity is less significant (maximum 45%), at higher initial concentrations of RIF (67.5 mg/L), the adsorption capacity increases by 206%, 184%, and 172% for fibers functionalized with 0.1, 0.5 and 1.0% alginate, respectively, compared to raw PET fibers.To explain this behavior, PET fibers functionalized with alginate (0.1, 0.5, and 1.0%) were examined using thermogravimetric analysis and SEM microscopy. The results indicate that as the concentration increases from 0.1 to 1.0%, alginate agglomerates on the surface of the PET fibers, which leads to a decrease in the number of functional groups available for the RIF adsorption process. Therefore, the functionalization of PET fibers with low concentrations of alginate (0.1%) allows obtaining an adsorbent with enhanced performance in the retention of RIF from aqueous solutions, and this is also a cost-effective approach.