An innovative method for reducing water sorption of injection‐molded polyamide 6 (PA6) bars by polymerization‐induced diffusion of styrene and grafting with polystyrene (PS) is reported. The process involves diffusion of styrene into PA6 bar in aqueous medium, addition of benzoyl peroxide to initiate polymerization of styrene, and further diffusion of styrene into the bar during polymerization. A hydrophobic PS‐rich shell consisting mainly of PS‐g‐PA6 can be formed in the surface layer of the PA6 bar, and as a result, the water sorption and dimensional change of PS‐modified PA6 bars reduce significantly. An incorporation of only 1.2% PS is sufficient, showing the advantage of this method over conventional melt mixing. The tensile modulus and strength of 1.2% PS‐modified PA6 bar increase slightly compared to those of neat PA6 bar due to reinforcement effect of rigid PS and reduced level of water‐caused plasticization, while maintaining the good ductility. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46243.