We performed variable-temperature synchrotron powder X-ray diffraction measurements and impedance spectroscopy under pressure for silver iodide (AgI) nanoparticles with a diameter of 11 nm. The superionic conducting alpha-phase of AgI nanoparticles was successfully stabilized down to at least 20 degrees C by applying a pressure of 0.18 GPa, whereas the transition temperature was 147 degrees C in bulk AgI at ambient pressure. To our knowledge, this is the first example of the alpha-phase of AgI existing stably at room temperature.