There has emerged a recognition that the submarine discharge of fresh, brackish, and marine groundwaters into the coastal ocean is comparable to the inputs via river discharge. However, the factors controlling submarine groundwater discharge (SGD) are poorly understood owing to a lack of SGD measurement tools in dynamic coastal waters. We observed bi-hourly variations of the natural SGD tracers, 222Rn and CH4, at a coastal seawater station over two seasons using novel monitoring techniques. This unique data set suggests that SGD increases sharply from neap to spring tide during the wet season. The observed increase is much greater than what would be expected from the rainfall and tidal-height changes. We conclude from this that the temporal variation of SGD is regulated predominantly by a semi-monthly fluctuation of a tidal oscillating pumping force in this environment.