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We used local P and S waves, and regional Lg waves to investigate the Mw 4.9 Offshore Jeju Island earthquake, whose records show evidence of a complex rupture. This earthquake provides a rare window to understand the seismogenesis of moderate‐sized earthquakes on the southern Korea–East China Sea continental shelf. We computed the relative source time functions (RSTFs) by aligning the signals on the origin time of the main and its empirical Green's function (EGF) events, allowing us to use them as differential times of the EGF pair. We determined subevent locations using direct‐wave RSTFs, and captured the rupture variability of the two large subevents using waveform inversion of stacked Lg‐wave RSTFs. The first subevent rupture started by two weak nucleation phases and propagated slowly and bilaterally. Then the second subevent ruptured westward. Our analysis demonstrates that the Lg‐wave train observed at regional distances is useful in investigating detailed slip history. Plain Language Summary: In‐depth studies of small‐to‐moderate‐sized earthquakes in offshore regions are often limited by sparse seismographic station coverage and a lack of close observations. Here, we demonstrate the effective use of local P and S waves and regional Lg waves to investigate the detailed rupture process of the 2021 Mw 4.9 Offshore Jeju Island, Korea earthquake. Lg wave is a guided S wave composed of a superposition of post‐critical reflections in the crust. Detailed seismological analyses using Lg waves revealed that earthquake rupture processes can be spatially complex even in areas with low seismic activity. Our analysis demonstrates that the Lg wave observed at distances greater than 150 km from the epicenter alleviates the limited local station coverage and can be a very useful signal to image the detailed slip history of the earthquake. Our analysis revealed the rupture complexity of the earthquake, expressed as a cascade of four sequential ruptures consisting of two small nucleation phases and two large subevents. Such detailed knowledge of earthquake rupture evolution is critical for understanding seismogenesis in this stable continental region setting. Key Points: We demonstrate an approach to capture the complex rupture process of an offshore moderate‐sized earthquake using limited observationsDirect P and S waves and Lg waves are used to image a spatiotemporal slip history of the Mw 4.9 Offshore Jeju Island, Korea earthquakeResults show a cascading rupture of four subevents on a fault, offering a deeper understanding of event mechanism in low‐seismicity regions [ABSTRACT FROM AUTHOR]