The 1946 Aleutian earthquake produced a tsunami of tsunami magnitude Ml=9.3, but the surface wave magnitude is only Ms=7.4, making it a tsunami earthquake. The discrepancy between the apparent size of the earthquake based on the seismic data and the size of the tsunami has been explained by several mechanisms, including a landslide and a slow earthquake, but there are few seismic data available to determine the correct mechanism. We study the generating mechanism of the tsunami using tsunami waveforms recorded on tide gauges. We have modeled the source of the 1946 Aleutian tsunami as the result of an underthrusting earthquake. We performed both forward and inverse modeling of the data using a finite difference calculation to compute synthetic tsunamis. We include both vertical and horizontal deformation of the ocean bottom due to faulting in the computation. The results of the inversion of the tsunami waveforms show that the slip on the fault is mainly concentrated in the shallow section of the fault near the epicenter. There is little slip in the area of the aftershocks, indicating that the aftershocks may represent an area of afterslip or triggered seismicity. The moment estimate of the earthquake is 23¿1020 N m, or Mw=8.2. The majority of the waveforms are well-explained by this fault model and slip distribution, including the Honolulu waveform, but the Hawaiian data also suggest that other effects, such as a landslide, may be necessary for explaining the abnormal tsunami amplitudes in Hawaii. The 1946 earthquake is similar to other tsunami earthquakes, such as the 1896 Sanriku and 1992 Nicaragua earthquakes, indicating that slow, shallow rupture may explain the disproportionally large tsunamis.¿ 1997 American Geophysical Union