Abstract The wave‐affected Marginal Ice Zones (MIZ) is a key region of air‐ice‐ocean interactions in the Southern Ocean (SO). However, challenges still remain for the large‐scale observation of the MIZ. In this study, we use four concurrent radar altimeters to retrieve the wave‐affected MIZ in the eastern Weddell Sea‐Indian Ocean sector, which was surveyed by an in situ campaign during July 2017. We show that a strong cyclone induced a 600 km‐wide MIZ in the eastern Weddell Sea, while in the Indian Ocean sector, MIZ width is generally 200 km throughout the month. The derived wave attenuation rate is closely related with both swell period and ice conditions. Moreover, we demonstrate that the MIZ width, combined with the incident swell power, serves as a proxy for ice thickness in the MIZ. The retrieval algorithms can be further applied to wave‐ice interaction studies and the construction of long‐term records of MIZs in the SO.