Harry Hammond Hess (May 24, 1906 – August 25, 1969) was an American geologist and a United States Navy officer in World War II who is considered one of the "founding fathers" of the unifying theory of plate tectonics.

He published theories on sea floor spreading, specifically on relationships between island arcs, seafloor gravity anomalies, and serpentinized peridotite, suggesting that the convection in the Earth's mantle is the driving force behind this process.

Hess accompanied Dr. Felix Vening Meinesz of Utrecht University on board the US Navy submarine USS S-48 to assist with the second U.S. expedition to obtain gravity measurements at sea.

[7][8] Hess joined the United States Navy during World War II, becoming captain of the USS Cape Johnson, an attack transport ship equipped with a new technology: sonar.

In a widely circulated report to the Office of Naval Research, he advanced the theory, now generally accepted, that the Earth's crust moved laterally away from long, volcanically active oceanic ridges.

[9][10] Seafloor spreading, as the process was later named, helped establish Alfred Wegener's earlier (but generally dismissed at the time) concept of continental drift as scientifically respectable.

The American Geophysical Union established the Harry H. Hess medal in his memory in 1984 to "honor outstanding achievements in research of the constitution and evolution of Earth and sister planets.