Highly flexible and electrically conductive materials have received considerable attention for their potential sensing applications, such as wearable devices. Piezoresistive materials can be used as strain sensors due to their ability to detect external loads by transforming the measured deformation to the changes of electrical resistance of the material. Compared to conventional metallic strain gauges, nanocomposite-based strain gauges have significantly large operational strain range and high sensitivities. Although cast molding is the traditional approach to fabricate certain nanocomposite strain gauges, additive manufacturing, usually referred to as the 3D printing, has been found to be rapid and economic way of manufacturing such strain gauges. In this study, a flexible piezoresistive strain gauge sensor is fabricated using a modified 3D printer. Polydimethylsiloxane polymer (PDMS) is chosen as the matrix material due to its high flexibility, and multi-walled carbon nanotubes (MWCNTs) are used as a conductive filler because of their excellent mechanical and electrical properties. The sensor is fabricated via direct 3D printing of the conducting patterns into a bath of uncured PDMS. Both fabricated features and sensing response of the material are investigated. Scanning Electron Microscopy (SEM) is used to characterize the microscale dispersion of MWCNTS within the conductive patterns. To study the sensing behavior, the strain sensor units are tested at cyclic tensile loads with various maximum strains. Finally, the manufactured sensors are employed as wearable sensors and attached on human skins to monitor human motions and estimate gestures.