Lightner Witmer (June 28, 1867 – July 19, 1956) was an American psychologist. He introduced the term "clinical psychology" and is often credited with founding the field that it describes. Witmer created the world's first "psychological clinic" at the University of Pennsylvania in 1896, including the first journal of clinical psychology and the first clinical hospital school in 1907.

Witmer contributed to numerous branches of psychology including school psychology. He contributed to the field of special education.

Little is known about Witmer's life. He is described as an introverted and private person.

Witmer was born in Philadelphia on June 28, 1867. His birth name was David L. Witmer Jr., but he changed it to Lightner at age 50. His parents were devout Catholics: his father, David Witmer, a Germantown, Philadelphia, pharmacist who graduated from a Philadelphia college in 1862, and his mother, Katherine Huchel, about whom little is known. He was the eldest of four children, followed by Albert Ferree, Lilly Evelyn, and Paul DeLancey. Later in life, Witmer became a faculty member at the University of Pennsylvania. Ferree earned his doctorate in physiology from the University of Pennsylvania, Lilly Evelyn received her bacteriology medical degree in Berlin, and Paul DeLancey earned a doctoral degree in pharmacy. By the end of 1905, Witmer and his siblings had all become doctors in a range of disciplines.

As a young man, Witmer wanted a better future and a better world after the social problems he saw as a result of the American Civil War. In 1880, Witmer and his brother Ferree enrolled in the Episcopal Academy of Philadelphia, a preparatory school and one of the best schools in America at the time.

Witmer showed his intelligence and reasoning ability at prep school. Witmer and two other boys were told to build a canoe, each having everything they needed to complete their task. His two schoolmates were arguing over who would build the canoe first, but Witmer thought and told the other children, "I wish to finish last as I will learn from others' mistakes and build the best canoe."

He graduated with high honors at age 17. In 1884, Witmer enrolled at the University of Pennsylvania to study art, but after a couple of years he transferred to the Finance and Economy Department, from which he obtained a bachelor's degree in 1888 (aged 20). During his first year, he was chosen as the class president and earned a reputation as an outstanding student. After a stint as a teacher, Witmer decided to return to the University of Pennsylvania for his graduate studies in political science.

During the fall of 1888, Witmer was offered a job as an instructor at Rugby Academy, a male secondary school. He taught history and English.