Gas hydrate is a new source of energy, which occurs both in marine sediments and in permafrost regions. Due to its vast amounts and potential economic and environmental significance, natural gas hydrates have increasingly attracted the attention of the scientific community. Until now, a vast amount of research work has been conducted regarding the occurrence, distribution, and formation mechanism of marine gas hydrates in the world’s ocean. However, the gas hydrate research and exploration in China is just starting. In the South China Sea, the geologic and tectonic settings, the temperature-pressure regime, and the methane-generating potential from the thick organic-rich sediments are all favorable for gas hydrate formation. Evidence for gas hydrate, such as the bottom-simulating reflector (BSR) on seismic-reflection profiles, has already observed in many regions in the South China Sea. Furthermore, initial geological, geophysical and geochemical investigations in the South China Sea have suggested great promise for the occurrences of gas hydrates. In this paper, we will discuss all available geochemical parameters that have been used and proved to be diagnostic tools for the search of gas hydrates. A number of geochemical anomalies may have related to the formation and dissociation of gas hydrates. They include gaseous hydrocarbon contents (mainly methane and ethane) in the marine sediments, carbon isotopes of the methane, downward and spatial profiles of Cl^-, SO₄^2- and major cations (e.g., Ca, Mg, Ba, Sr, B, and NH₄) contents, and ¹⁸O, D, ¹¹B, and ⁸⁷Sr/⁸⁶Sr ratios of the pore waters. There are also many other potential indicators for the gas hydrates, such as thermoluminescence anomaly, reflectance spectra using diffuse reflectance spectrometry, and biogeochemical evidence. It is suggested that with the progress in theoretical and experimental study and intensive geophysical and geochemical explorations, we will finally target the gas hydrate occurrences in the South China Sea in the near future.