Elmer Wayne Henley Jr. (born May 9, 1956) is an American serial killer and accomplice to murder convicted in 1974 of the murder of six of the twenty-nine known victims of the Houston Mass Murders, which occurred in Houston and Pasadena, Texas, between 1970 and 1973.

One of two known accomplices to Dean Corll, Henley initially solely assisted Corll in the abduction of the victims before gradually and increasingly participating in their torture, murder and burial. He shot Corll to death on August 8, 1973, when he was seventeen years old, before divulging his knowledge of and participation in the crimes to authorities.

Tried in San Antonio, Henley was convicted of six murders and sentenced to six consecutive terms of 99-years' imprisonment. He was not charged with the death of Corll, which prosecutors had previously ruled had been committed in self-defense. Henley did successfully appeal his conviction, although he was again convicted of six murders in June 1979. He is currently incarcerated within the Telford Unit in Bowie County, Texas.

At the time of the discovery of the crimes, the case was considered the worst example of serial murder in United States history.

Elmer Wayne Henley Jr. was born on May 9, 1956, in Houston, Texas, the eldest of four sons born to Elmer Wayne Henley Sr. (1938–1986) and Mary Pauline Henley (née Weed) (b. 1937). His parents were both in their mid-teens at the time of his birth, and the couple initially lived with Mary's parents in Houston Heights until they could afford their own home following his father finding employment as a stationary engineer.

As a child, Henley was an avid reader and both an attentive and academically achieving student whose grades saw him typically in the top quarter of his class. He was also markedly religious, and briefly held an aspiration to become a preacher. As his mother and grandparents were devout Christians, this religious devotion was encouraged.

Henley's father was an alcoholic and adulterer who physically assaulted his wife and sons, and the children were largely raised by their mother and maternal grandparents. As a child, Henley strove to protect his mother from his father's violence; she in turn was markedly protective of her children, and often shielded them from her husband's violence. On one occasion as an adolescent, Henley observed his father striking his mother before pushing her into a corner to continue his assault; he successfully prevented his father from further hitting his mother by pointing a shotgun at his father and shouting, "Drop it, Dad!" Although his memories of his father were conflicted, he would later fondly remark of his early childhood: "I have memories of [my father] walking me to school, and of Cub Scout and Boy Scout activities. I went to work with him, and he'd tell me about boilers and air conditioners."

Although Henley's father mistreated and neglected his family, he was present throughout his sons' childhood and early adolescence, and despite the belittling he endured from his father, Henley strove to meet his approval. Although occasionally bullied at school from the fifth grade onward, he was popular among many of his peers—both male and female—and was attracted to the attitudes of the contemporary hippie movement of the late 1960s and early 1970s.