The fundamental relationship between electronic transport properties and applied strain in piezoresistive materials permits their usage in a broad range of applications related to electromechanical systems. Furthermore, strain engineering has been demonstrated as a useful technique to improve the performance of electronic devices, as it allows for enhanced carrier mobility and direct control of the band gap of semiconductors. Quantitative characterization of the relationship between electrical properties and strain is essential to the development of piezoresistive-based devices and to the application of strain-engineered devices. To quantify fundamental piezoresistive properties and reach high strain levels, testing at the nanoscale is critical, as the size scales can fall below the threshold for external or bulk defects. However, proper characterization of the piezoresistivity is a challenge at the nanoscale due to the miniscule specimen size. In this work, in situ tensile tests inside a scanning electron microscope (SEM) are conducted to study the mechanical and electromechanical properties of tellurium (Te) and germanium (Ge) nanowires. The in situ tensile testing technique allows real-time observations of elastic/plastic behaviors and gives the resistance changes under different strains for the determination of the piezoresistivity. The reversibility and the repeatability of the observed piezoresistive properties can be confirmed from loading-unloading tests and measurements of multiple nanowires with different diameters. These tests offer accurate experimental data that yields insights into the fundamental mechanisms behind piezoresistivity, such as strain-induced changes in band structure.