Abstract Analysis of high resolution sub-bottom profiles, topographical data and sediment samples reveals that the tidal delta system of Qiongzhou Strait is morphologically a “butterfly delta” system consisting of Western Mouth Tidal Delta (WMTD) and Eastern Mouth Tidal Delta (EMTD). WMTD covers an area of about 11,000km2, and is more than two times larger than EMTD (4000km2). WMTD is fan-shaped with a tributary branch extending southwards along Hainan Island, while EMTD is typically fan-shaped. Seismic architectures of WMTD and EMTD are both characterized by a thick progradational unit overlying a thin transgression unit. Both WMTD and EMTD start to develop at about 8.7calkaBP when Qiongzhou Strait completely opened, and have the same three sedimentary facies: tidal delta top, tidal delta slope and tidal delta toe. The formation of WMTD and EMTD is attributed to Qiongzhou Strait's hydrodynamic regime which is dominated by the strong east–west reciprocal-tidal currents and influenced by the year-round westward mean flow modified by the wind-driven seasonal currents.