In this letter, we demonstrate a fully inkjet-printed electrochemical lactate sensor. The sensor is inkjet-printed on a flexible substrate, Kapton film, where silver nanoparticles (Ag NPs) are printed as conductive electrodes. The working and reference electrodes of the sensor are Ag NPs/NiO NPs/Polyurethane (PU) and Ag/AgCl(s)/PU layers respectively. The inkjet-printed PU film can not only prevent the annihilation of the Ag NPs resulting from the dissolution of Ag and the formation of AgCl in the phosphate-buffered saline (PBS) but also stabilize the lactic acid measurements by maintaining a potential variant within 2% at different lactate concentrations. The as-printed sensor exhibits a linear sensitivity of 8.86nA/mM/mm2 (R=99.6%) in the range from 0.6 to 2.2 mM. Clinical blood plasma testing results show that the trends of lactic acid change in patients measured by the sensor are the same as the ones measured using the existing detection method, colorimetric assays, in a tertiary university-affiliated hospital.