This study proposes the detection of large offshore tsunamis by airborne radar. Altimeter measurements were carried out south of Japan by an observational aircraft along the Jason satellite tracks crossing over the Kuroshio Current. A nadir-pointing frequency-modulated continuous wave (FM-CW) radar and a global navigation satellite system (GNSS) based positioning system were concurrently used to measure the sea surface height (SSH). The SSH was validated against the Jason-2 and Jason-3 satellite altimeter SSH. After the removal of the geoid and tidal signals, the observation error was less than 10 cm. This is sufficiently small to capture offshore tsunamis within 10 min after the genesis. This was demonstrated by virtual airborne radar-detected SSH from a numerically simulated hypothetical large offshore tsunami south of Japan. The feasibilities and limitations of utilizing commercial airplanes with altimeters to create a dense tsunami observation network are discussed. The suggested methodology can be adapted to various emerging airborne platforms such as unmanned aerial vehicles. Moreover, a dense coverage of SSH observations is invaluable from an oceanographic point of view.