Newly-acquired seismic data reveal widespread carbonate deposits covering a large part of the northwestern South China Sea margin. Three carbonate platforms are identified to have developed on the topographic highs inherited from tectonic deformation and volcanic accretion. Across the carbonate platforms, the Miocene strata are characterized by high-amplitude seismic reflections and distinct platform architecture that overlaps older strata. The Guangle and Xisha carbonate platforms grew on faulted blocks due to South China Sea continental rifting, while the Zhongjian carbonate platform developed on a structural high induced by early Miocene volcanism. During the late Miocene, partial drowning resulted in the inhibition of platform growth, eventual platform drowning and termination of most carbonate deposition. The drowning of the Guangle and Zhongjian carbonate platforms is shown by the supply of siliciclastic sediments during the late Miocene and seems to be closely linked to late Neogene volcanic activity, whilst the drowning of the Xisha carbonate platform is primarily related to relative eustatic changes. Our results imply that tectonic activity, volcanism and eustasy are the dominant controls on the evolution of carbonate platforms on the northwestern margin of the South China Sea.