Understanding the forces and magma system dynamics on timescales of seconds to minutes remains challenging. In the January 2022 phreatoplinian Hunga Tonga‐Hunga Ha'apai eruption, four remarkably similar seismic subevents within a 5‐min interval occurred during the intensifying early eruptive phase. The subevents are similar in waveforms and durations (∼25 s each). Each subevent begins with an unusual negative P‐wave polarity which is inferred, using full‐wave seismic modeling, to be caused by an upward single‐force mechanism at the volcano created by a magma hammer likely in response to magma flow blockage/constriction during the early part of the eruption as discharge rapidly increased over orders of magnitude with concomitant conduit geometry evolution and instability. Our proposed episodic magma hammer model is consistent with thermodynamic and phase properties of the magmatic mixture, and yields an estimate of conduit mass flow in agreement with vent discharge rates derived from satellite imagery of plume heights. Plain Language Summary The seismic record of the 15 January 2022 Hunga Tonga‐Hunga Ha'apai explosive eruption exhibited a remarkably regular pattern, recording repeating volcanic processes. Within an interval of ∼300 s during an early eruptive phase, four strong seismic subevents occurred and were recorded by global seismic stations. Detailed seismic analyses showed that each of these subevents is similar in waveform and duration and is characterized by a sequence of four forces: upward, downward, upward, and downward. We suggest that the first upward seismic force at the volcano was likely created by an ascending magma colliding with a blockage or conduit constriction that occurred during and because of the ongoing eruption. We attribute the other forces to upward magma backflow and piston motion in the conduit, owing to their similar time durations. The magma hammer mechanism allows us to estimate the magma flow rate in the subsurface conduit, which is consistent with the vent discharge rate observed by satellite imagery. Key Points The 15 January 2022 Hunga Tonga‐Hunga Ha'apai eruption had four episodic seismic subevents with similar waveforms within ∼300 s An unusual upward force jump‐started each subevent A magma hammer explains the force and estimates the subsurface magma mass flux which fits the vent discharge rate based on satellite data