In this study, electrospun nanofibers of poly (vinyl alcohol) (PVA) and styrene–butadiene–styrene triblock copolymer (SBS) were employed in conventional water-swellable rubber (WSR) to design WSR composites with improved water swelling and mechanical properties. With the introduction of PVA nanofibers, considerable improvement in elasticity, strength, and water-swelling behavior was observed. After immersion, PVA nanofibers dissolved within the composite to in situ form water channels to connect isolated super-absorbent polymers (SAPs). Those water channels led to an increase in water uptake by the WSR composite. Furthermore, the secondary water-swelling behaviors of the WSR composite showed a remarkable increase in swelling rate as well as in mechanical properties. The addition of SBS nanofibers had a marked impact on the mechanical properties of the WSR composite. Their roles became more pronounced after water immersion. The proposed enhancement mechanism is also discussed.