Orrin Hendren Pilkey Jr. (September 19, 1934 – December 13, 2024) was an American marine geologist who was Professor Emeritus of Earth and Ocean Sciences, Nicholas School of the Environment, at Duke University, and founder and director emeritus of the Program for the Study of Developed Shorelines (PSDS) based at Western Carolina University.

Pilkey's research centers on both basic and applied coastal geology, focusing primarily on barrier island coasts and the effects of shoreline stabilization and development, and sea level rise.

In general, Pilkey argues that mathematical models cannot be used to accurately predict the behavior of beaches, although they can be useful if directional or orders-of-magnitude answers are sought.

In the book, Useless Arithmetic, written with his daughter, Linda Pilkey-Jarvis, they argue that the outcome of natural processes in general cannot be accurately predicted by mathematical models.

Retreat from a Rising Sea, written with daughter, Linda, and son, Keith, highlights the need for moving back from the coast, a process that is already beginning in some parts of the world, such as Arctic barrier islands, atolls, and river deltas.