A neutral particle analyzer was designed and implemented for plasma diagnostics on the spherical tokamak ST40. The diagnostic aims to measure both the bulk ion temperature (0.5–10 keV) and the energy distribution of fast ions generated by a neutral beam injection (up to 40 keV). Its key features comprise the ability to separate hydrogen isotopes (protium and deuterium), a high temporal resolution of under 1 ms, a low threshold for the measured ion temperature, and an integrated calibration ion source to monitor the stripping foil condition and detection channel sensitivity. A detailed description of the analyzer’s design is provided, along with the procedures for determining the instrumental functions of its channels and for reconstructing the plasma ion temperature from the measurement results. An example of its application for plasma temperature measurements on the ST40 tokamak is also presented.