The Yinggehai Basin (YB) in the northwest of the South China Sea (SCS) has preserved the complete evolution of the Red River Fault (RRF), whose motion over time has largely contributed to shaping the current tectonic framework of the southeastern Tibetan Plateau and Indochina Block. Here we, for the first time, image the RRF, crustal architecture, and crust‐mantle boundary (i.e., Moho) in detail under the YB from two high‐resolution 40‐km‐depth seismic reflection pre‐stack migration profiles. Our seismic reflection images provide direct seismic evidence to support a deep‐penetrating RRF, most likely crosscutting the Moho. In this sense, the RRF was a large‐scale shear zone, along with the clockwise rotation of the Indochina Block, to accommodate the Indochina Block extrusion during Oligocene‐early Miocene and form the YB. We highlight that the material extrusion along deep‐penetrating faults dominated the Tibetan Plateau deformation in response to the India‐Asia collision at an early stage. Plain Language Summary The India‐Asia collision has not only created the highest Tibetan Plateau but also resulted in material extrusion outward along multiple large‐scale strike‐slip faults. The Red River Fault (RRF) is suggested to be one of these large‐scale strike‐slip faults. The geometry and deep extension of these strike‐slip faults are fundamental to understanding the manner of the material extrusion. Despite many studies describing geophysical and geological observations of the RRF, its nature in response to the India‐Asia collision remains unknown. The Yinggehai Basin (YB) northwest of the South China Sea with >10 km thick sedimentary has preserved complete evolution of the RRF. In this study, we use high‐resolution deep seismic reflection data with a depth of 40 km, for the first time, to map the crustal architecture, detailed geometries of the crust‐mantle boundary (i.e., Moho) and the RRF under the YB. The seismic reflection images indicate that the RRF is a deep‐penetrating fault that could serve as a large‐scale shear zone. We further highlight that the RRF played a vital role in moving the Indochina Block away from the Tibetan Plateau in response to the India‐Asia collision at an early stage, and subsequently stretching and thinning the continent, eventually forming the YB. Key Points For the first time, we image a reflected Moho cut by the Red River Fault (RRF) structure under the Yinggehai Basin (YB), South China Sea The RRF in the YB is inferred to be a deep‐penetrating fault, most likely cutting through the Moho boundary The RRF was inferred to be a large‐scale shear zone accommodating the Indochina Block extrusion during Oligocene‐early Miocene