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A tsunami was triggered in the North Pacific Ocean by the 2021 Alaska earthquake (M 8.2) on July 29. We studied the source properties and resonance characteristics of the tsunami event using observed records from offshore tsunameters and coastal tide gauges as well as numerical simulation. Spectral analyses were conducted on records at tsunameters and tide gauges. We reconstructed the tsunami source spectrum by calculating the ratio of tsunami spectra to the background spectra of these stations. Based on the source spectrum, we estimated the source size of the 2021 Alaska earthquake to be 332 km (length) × $\times $ 256 km (width), which was generally consistent with the source model proposed by the United States Geological Survey using seismological approaches. In addition, we also performed spectral analyses of the tsunami wavefield in a region of interest, including the Aleutian Islands and the Alaska Peninsula. Standing wave systems were found south of the Alaska Peninsula and the Aleutian Islands. The Aleutian Islands prevent the propagation of tsunami oscillations. Only a small portion of energy propagates through the archipelago from the main part of the Pacific Ocean to the Bering Sea. Plain Language Summary: On 29 July 2021, an earthquake occurred offshore the Alaska Peninsula and generated a tsunami. It was the largest earthquake event in the United States in the past 50 years. We analyzed the tsunami records of offshore and coastal stations. We estimated the size of the source fault merely based on tsunami data, as a complementary method to seismological approaches. Moreover, we also investigated tsunami resonance characteristics in the Aleutian Islands and the Alaska Peninsula. Tsunami resonance is a phenomenon that occurs when the period of the tsunami matches the oscillation periods of bays or the continental shelf. We found that the tsunami amplitudes were amplified in the Aleutian Islands at certain periods owing to resonant oscillations. Key Points: Tsunami source spectrum was reconstructed to yield an estimation of the tsunami source size of the 2021 Alaska earthquakeSix oscillation modes in the Aleutian Islands and the Alaska Peninsula were identified by modal analysis, ranging from 120.5 to 46.5 minThe Aleutian Islands hinder the propagation of tsunami oscillations from the main part of the Pacific Ocean to the Bering Sea [ABSTRACT FROM AUTHOR]