In recent times, flexible piezoresistive polymer nanocomposite-based strain sensors are in high demand in wearable devices and various new age applications. In the polymer nanocomposite-based strain sensor, the dispersion of conductive nanofiller remains challenging due to the competing requirements of homogenized dispersion of nanofillers in the polymer matrix and retaining of the inherent characteristics of nanofillers. In the present work, waterproof and flexible poly(vinylidene difluoride) (PVDF) with a polymer-functionalized hydrogen-exfoliated graphene (HEG)-based piezoresistive strain sensor is developed and demonstrated. The novelty of the work is the incorporation of polystyrene sulfonate sodium salt (PSS) polymer-functionalized HEG in a PVDF-based flexible piezoresistive strain sensor. The PSS-HEG provides stable dispersion in the hydrophobic PVDF polymer matrix without sacrificing its inherent characteristics. The electrical conductivity of the PVDF/PSS-HEG-based strain sensor is 0.3 S cm-1, which is two orders of magnitude higher than the PVDF/HEG-based strain sensor. Besides, near the percolation region, the PVDF/PSS-HEG shows a maximum gauge factor of 10, which is about two times higher than the PVDF/HEG-based flexible strain sensor and 5-fold higher than the commercially available metallic strain gauge. The enhancement in the gauge factor is due to the stable dispersion of PSS-HEG in the PVDF matrix and electron conjugation caused by the adherence of negatively charged sulfonate functional groups on the HEG. The developed waterproof flexible strain sensor is demonstrated using portable wireless interfacing device for various applications. This work shows that the waterproof flexible PVDF/PSS-HEG-based strain sensor can be a potential alternative to the commercially available metallic strain gauge. Copyright © 2020 American Chemical Society.