The 15 January 2022 Tonga eruption seemed to have caused a strong depletion in the ionospheric electron density. However, the eruption occurred during a moderate geomagnetic storm, so that the depletion could be a local negative effect of the storm. In this work, for the first time, we analyze this depletion and discuss relative contributions of the eruption and the storm through measurements of GNSS‐derived vertical total electron content (VTEC), O/N2 ratio by TIMED/GUVI, ion density and temperature by ICON/IVM, and simulations by Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). We show that shortly after the eruption onset the VTEC in the vicinity of the volcano dropped by ∼80%–95% below the quiet‐time values. The depletion extended up to 4,000 km away from the volcano and lasted for ∼16 hr, that is, until local morning hours. Our results suggest that the depletion was initially caused by the eruption (60%–75% negative deviation) and was further reinforced by the storm, by at least 20%. Spatially, only ∼1,500 km could be attributed to the eruption. This study provides a good illustration of understanding the contributions of forcings from above and below to the ionosphere.