Poly(lactide) (PLA) is an eco-friendly and low-cost polymer with a shear piezoelectricity easily induced by uniaxial stretching and without poling process. These interesting features make PLA as a good candidate to replace the widely-used poly(vinylidene difluoride) (PVDF) piezoelectric polymer, in particular for flexible energy harvesting applications aiming to power wearable sensors. In this work, PLA films were fabricated by a simple extrusion–orientation process. The energy harvesting efficiency of the PLA films were characterized using a custom-built test bench designed for flexible piezoelectric materials and working in tensile mode at low strain frequencies. The measurements on PLA films show a maximal output power about 3.7 μW harvested by applying a strain of 1% at 20 Hz. A theoretical model was developed based on electromechanical parameters to ascertain the energy harvesting performances of PLA. The validated model was also helpful to estimate the output power from PLA films by increasing their active surface areas and/or improving their shear piezoelectric coefficients (d14). The optimized PLA film could produce an output power comparable to that produced by PVDF film in the same mechanical excitation conditions. This study consequently highlights the potential of PLA for energy harvesting applications and the utility of the developed model to estimate the electrical power generated by piezoelectric polymers.