American seismologist and glaciologist who in 1911 developed the elastic rebound theory of earthquake mechanics, still accepted today.

Reid was professor of applied mechanics at Johns Hopkins University, Baltimore, from 1896 until he became emeritus professor in 1930. His early career was mainly concerned with the study of the structure, composition, and movement of glaciers (glacier). Later he became involved in the study of earthquakes and earthquake-recording devices. He was first to develop a mechanism that explained how earthquakes were a result of faulting and not the reverse. He wrote an analysis of the 1906 San Francisco earthquake as part of the California State Earthquake Investigation Commission report, Mechanics of the Earthquake.