The Yonaguni Monument is a massive underwater rock formation off the coast of Yonaguni, the southernmost of the Ryukyu Islands, in Japan. While many archaeologists and geologists believe that it is a natural formation, some hold the view that some of its features are man-made. The sea off Yonaguni is a popular diving location in winter due to its large population of hammerhead sharks.

In 1987, while looking for a good place to observe the sharks, Kihachiro Aratake, a director of the Yonaguni-Cho Tourism Association, noticed some singular seabed formations resembling architectonic structures. Shortly thereafter, a group of scientists directed by Masaaki Kimura of the University of the Ryūkyūs visited the formations. Kimura is a strong advocate of the view that the formations are artificial.

The formation has since become a relatively popular attraction for divers, in spite of the strong currents. In 1997, Japanese industrialist Yasuo Watanabe sponsored an informal expedition comprising writers John Anthony West and Graham Hancock, photographer Santha Faiia, geologist Robert Schoch, a few sport divers and instructors, and a shooting crew for British Channel 4 and Discovery Channel. Another notable visitor was freediver Jacques Mayol, who wrote a book on his dives at Yonaguni.

A plaque in his honor was fixed to the undersea formations after his suicide in 2001. The Monument consists of medium to very fine sandstones and mudstones of the Lower Miocene Yaeyama Group, deposited about 20 million years ago. Most of the significant formations are connected to the underlying rock mass (as opposed to being assembled out of freestanding rocks).

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