William Thierry Preyer (July 4, 1841 - July 15, 1897) was an English-born physiologist who worked in Germany.
Preyer was born in Rusholme at Manchester. He studied physiology and chemistry at Heidelberg, where he received his doctorate in 1862. In 1866 he earned his medical degree at the University of Bonn, and in 1869 succeeded Johann Nepomuk Czermak as professor of physiology at the University of Jena. At Jena he was also director of the Physiology Institute.
Preyer was a founder of scientific child psychology, and a pioneer concerning research of human development that was based on empirical observation and experimentation. He was inspired by Charles Darwin's theory of evolution and Gustav Fechner’s work in psychophysics. He published two landmark works; a book on developmental psychology called Die Seele des Kindes (The Soul of the Child), and a book on developmental physiology titled Specielle Physiologie des Embryo (Special Physiology of the Embryo). Both works laid the foundation in their respective disciplines regarding future study of modern human development.
At Jena, Preyer introduced experimental-scientific training methods into his lectures, and also created seminars on physiology. Today, the "William Thierry Preyer Award" is issued by the European Society on Developmental Psychology for excellence in research of human development.