Tsunamis often travel long distances without losing power and severely devastate some coastal areas while leaving others with little damage. This unpredictable situation has been a major challenge for accurate and timely tsunami forecasting to facilitate early-warning and possible evacuations of affected coastal communities without disturbing the lives of others. Here we show evidence from satellite altimetry observations of the 2011 Tohoku-Oki earthquake-induced tsunami that sheds light on this issue. Three satellites observed the same tsunami front, and for the first time, one of them recorded a tsunami height about twice as high as that of the other two. Model simulations, based on the GPS-derived earthquake source and constrained by measurements of seafloor motions near the hypocenter, confirm that the amplified tsunami is one of several jets formed through topographic refraction when tsunamis travel along ocean ridges and seamount chains in the Pacific Ocean. This process caused the tsunami front to merge as it propagates, resulting in the doubling of the wave height and destructive potential in certain directions. We conclude that the potential of merging tsunamis should be emphasized in mapping tsunami hazards and assessing risk levels at key coastal facilities.