Abstract: This study investigates co-seismic ionospheric disturbances (CSIDs) triggered by the Mw 7.7 strike-slip earthquake in Myanmar on 28 March 2025, with a focus on how complex source rupture processes influence the earthquake–atmosphere–ionosphere coupling. High spatiotemporal resolution total electron content (TEC) data from the ground-based GNSS networks IONISE and IGS are employed. CSID signals are effectively extracted through sliding-window detrending and a fourth-order Butterworth band-pass filter with a period band of 2–6 minutes. Results show that during 06:36–06:39 UTC, a clear “N”-shaped disturbance with a dominant period of approximately 5 minutes and a peak amplitude of 0.97 TECU emerged in the near-field region (<1000 km from the epicenter). The disturbance exhibits pronounced east-west azimuthal asymmetry, with significantly larger amplitudes in the eastern sector (azimuth 30°–225°) compared to the west. This asymmetry is consistently observed across multiple satellites with diverse viewing geometries—including near-zenith and geostationary satellites—ruling out line-of-sight geometric effects or geomagnetic modulation as primary causes. The horizontal propagation velocity of the CSID ranges from 1.6 to 3.4 km/s, showing clear azimuthal dependence and aligning closely with surface Rayleigh wave speeds, indicating that the disturbance is predominantly excited by the Rayleigh wave mode. This azimuthal asymmetry is reasonably explained by the earthquake’s rupture dynamics—initial bilateral propagation followed by a unidirectional southward supershear rupture—which enhances Rayleigh wave radiation in specific directions via Mach wave effects. This study demonstrates that GNSS-derived TEC observations can effectively capture the directionality and dynamic details of coseismic rupture, providing critical observational constraints for characterizing Rayleigh wave radiation patterns and advancing our understanding of cross-layer coupling processes.