Dennis Lynn Rader (born March 9, 1945), better known as the BTK killer, is an American serial killer who murdered at least ten people in Wichita and Park City, Kansas, between 1974 and 1991. Although he occasionally killed or attempted to kill men and children, Rader typically targeted women. His victims were often attacked in their homes, then bound, sometimes with objects from their homes, and either suffocated with a plastic bag or manually strangled with a ligature.

In a series of crimes that terrorized Wichita in the mid-to-late 1970s, Rader also initiated a series of taunting letters he sent to police and media outlets, describing his crimes in detail and referring to himself as BTK (for "bind, torture, kill"). In addition, he stole keepsakes from his female victims, including underwear, driver's licenses and personal items. In 1979, BTK suddenly went quiet, and despite an exhaustive investigation, the case grew into one of the most infamous cold cases in American history. Rader would later confess to killing three further victims between 1985 and 1991 that were not initially linked to BTK but were confirmed to be his doing through DNA and items found in his possession.

In 2004, after a thirteen-year hiatus, Rader resumed sending letters, where he hinted at committing further crimes. Based on items he turned over to law enforcement, he was identified and arrested in February 2005, pleading guilty to his crimes months later and given ten consecutive life sentences. He is currently incarcerated at the El Dorado Correctional Facility.

Dennis Lynn Rader was born in Pittsburg, Kansas, on March 9, 1945, the oldest of four sons. His parents were bookkeeper Dorothea Mae Rader (née Cook) and Kansas Gas Service worker and former U.S. Marine William Elvin Rader. His parents were members of the Zion Lutheran Church in Pittsburg, where Rader was baptized. Both of his parents worked long hours and paid little attention to their children at home, and in later years Rader said he felt neglected. Rader's father was described as strict and God-fearing but not abusive.

Growing up in Wichita, Rader had an interest in novels, comic books, fishing and cub scouting. From a young age, however, he also harbored sadistic sexual fantasies about torturing "trapped and helpless" women. Rader exhibited zoosadism by torturing, killing and hanging small animals. He acted out sexual fetishes for voyeurism, autoerotic asphyxiation and cross-dressing, often spying on female neighbors while dressed in women's clothing, including women's underwear that he had stolen. He also masturbated with ropes or other bindings around his arms and neck.

As a student at Wichita Heights High School, Rader, who did not play any sports nor partake in any clubs, was described by fellow students as utterly lacking humor and always "hung back in the background." He spent most of his out-of-school time working as a bag boy and shelf stocker for a local grocery store to earn money for a motor vehicle. After graduating in 1963, Rader attended Kansas Wesleyan University, where he received only mediocre grades and dropped out after one year.

Rader joined the United States Air Force in June 1966. He completed basic training and technical school in Texas before being stationed at Brookley Air Force Base in Mobile, Alabama, where he worked as an antenna installer and maintenance specialist. After being stationed for assignments in Turkey, Greece and South Korea, Rader was transferred to Japan in January 1968, where he spent six months stationed at Kadena Air Base in Okinawa followed by twenty-five months stationed at Tachikawa Airfield in Tokyo. He earned a medal for good conduct and a ribbon for marksmanship; he also earned a National Defense Service Medal because his service coincided with the military being under national emergency due to the Vietnam War. Rader later credited the Air Force for teaching him knowledge about sex.

Rader received an honorable discharge on August 12, 1970, at the rank of staff sergeant. He subsequently served in the Air Force Reserve Command until June 20, 1972, around the time he was hired for the Coleman Company. A year after his discharge, Rader purchased a home in Park City, Kansas, a suburb of Wichita. He enrolled at Butler County Community College in El Dorado, earning an associate degree in electronics engineering in 1973. He then attended Wichita State University and graduated in 1979 with a Bachelor of Science degree, majoring in administration of justice.