An electrostatic probe measurement has been established to measure the electron temperature, ion temperature, and electron density within an expanding plasma jet. Due to the plasma being in a transitional hydrodynamic regime, neither collisionless nor collisional probe theories could be directly applied to the present case. Therefore, an inverse method was used in which the shape of the current–voltage curve of the probe was iteratively calculated for given plasma conditions and directly compared with the measured result. Using this method, the temperatures and densities could be estimated even though the established Langmuir probe theory was not applicable.