On-board satellites techniques provide global coverage and could play an important role in the continuous oceanic survey to prevent the damage produced by powerful tsunamis. We report here new ionospheric observations related to three significant transpacific tsunami events triggered by the 2006 Kuril earthquake, the 2009 Samoa earthquake and the 2010 Chile earthquake. Total Electron Content (TEC) variations extracted from data recorded by a dense Global Positioning System (GPS) network based in Hawaii show ionospheric disturbances within the hours following the tsunami wave passage at sea-level. For each event, we observe ionospheric gravity waves propagating with velocity, direction and arrival time coherent with the tsunami. The tsunamigenic signature in the ionosphere is also compared to in-situ sea-level measurements. These observations provide new examples of the sensitivity of the ionosphere to tsunamigenic gravity waves and confirm that ionospheric monitoring by GPS can provide complementary information on tsunami propagation.