Harry Hammond Hess (1906-1969) was an American geologist. He is best known for his theories on sea floor spreading, specifically work on relationships between island arcs, seafloor gravity anomalies, and peridotite, suggesting that the convection of the earth's mantle was the driving force behind this process.

This work provided a conceptual base for the development of the science of plate tectonics.

Hess was also involved in the Mohole project (1957 - 1966), an investigation into the feasibility and techniques of deep sea drilling.