Wearable systems are becoming highly attractive in different application areas. Recently, particular attention has been focused on the development of personal portable devices for monitoring occupational safety and health. Smart clothes based on strain sensors integrated on fabric seem to be a promising solution for real-time measurement of physiological endpoints. However, the development through a simple and cost-effective process of smart textiles characterized by high sensitivity, wearability and stable response even during physical activity, in case of exposition to environmental conditions and after washing is still challenging. In this work, the authors have developed a novel strain sensor made of graphene nanoplatelets (GNPs) properly dispersed into a water-based transparent ink, then deposited via screen printing technique on a synthetic fabric. Rheological investigations of the GNP-filled inks, morphological characterization of coated fabrics, electrical measurements of films obtained with different GNPs concentrations were performed. Smart textile specimens loaded with 3%wt and 3.8%wt of GNPs-based inks were characterized through quasi-static tensile tests to investigate the electromechanical response, even after a washing cycle. Specimens have shown a sensitivity of about 30 for a strain of 5%. This performance is interesting for different applications such as monitoring of respiratory and heart rates.