The impact of tsunamigenic earthquakes upon the Earth’s outer shells provides additional information about the advance approach of tsunamis to infrastructure facilities. This paper examines the measured impact of atmospheric internal gravity waves generated by tsunami propagating after the Kamchatka earthquake on July 29, 2025, upon the ionosphere. The measurements were made at a considerable distance from the earthquake’s epicenter (in the Hawaiian Islands). Tidal tsunami monitoring stations recorded the tsunami’s arrival, while variations in the ionospheric total electron content were recorded via GPS. Observations have shown that the arrival of tsunami-driven atmospheric waves significantly precedes the arrival of sea waves at the observation point. Attention is drawn to the specific type of variations of the total electron content in the ionosphere and their spectral features, which clearly indicate the early arrival of atmospheric waves at the observation point. Observations have shown that the characteristics of the ionosphere’s response to gravity waves generated by sea waves can be used in a tsunami early warning system.