Tsunami waves propagating across long distances in the open-ocean can induce atmospheric gravity waves by dynamic coupling at the surface. In the period range 10 to 20 minutes, both have very similar horizontal velocities, while the gravity wave propagates obliquely upward with a vertical velocity of the order of 50 m s−1, and reaches the ionosphere after a few hours. We use ionospheric sounding technique from Global Positioning System to image a perturbation possibly associated with a tsunami-gravity wave. The tsunami was produced after the Mw= 8.2 earthquake in Peru on 2001 June 23, and it reached the coast of Japan some 22 hours later. We used data from the GEONET network in Japan to image small-scale perturbations of the Total Electron Content above Japan and up to 400 km off shore. We observed a short-scale ionospheric perturbation that presents the expected characteristics of a coupled tsunami-gravity wave. This first detection of the gravity wave induced by a tsunami opens new opportunities for the application of ionospheric imaging to offshore detection of tsunamis.