The Slenderman is a fictional supernatural character that originated as a creepy pasta meme created by something awful forums user Eric Knudsen (also known as "Victor Surge") in 2009. It is depicted as resembling a thin, unnaturally tall man with a blank and usually featureless face, wearing a black suit.

Stories of the Slender Man commonly feature him stalking, abducting or traumatizing people, particularly children. The Slender Man is not confined to a single narrative, but appears in many disparate works of fiction, typically composed online. Fiction relating to the Slender Man encompasses many media, including literature, art and video series such as Marble Hornets. Outside of online fiction, the Slender Man has had impact on popular culture, having been referenced in the video game Minecraft generated video games of his own, such as Slender: Eight pages and Slender: The Arrival.

Beginning in 2014, a minor moral panic occurred over the Slender Man after readers of his fiction were connected to several violent acts, particularly near-fatal stabbing of a 12-year-old girl in Waukesha, Wisconsin.

The Development of an Internet Mythology, Professor Shira Chess of the University of Georgia connected the Slender Man to ancient folklore about fairies. Like fairies, Slender Man is otherworldly, with motives that are often difficult to grasp; like fairies, his appearance is vague and often shifts to reflect what the viewer wants or fears to see, and, like fairies, the Slender Man calls the woods and wild places his home and kidnaps children.

the Slender Man's fictional "mythology" has evolved without an official "canon" for reference, his appearance, motives, habits, and abilities are not fixed, but change depending on the storyteller. He is most commonly described as very tall and thin with unnaturally long, tentacle-like arms (or merely tentacles), which he can extend to intimidate or capture prey. In most stories his face is white and featureless, but occasionally his face appears differently to anyone who sees it. He appears to be wearing a dark suit and tie. The Slender Man is often associated with the forest and/or abandoned locations and has the ability to teleport. Proximity to the Slender Man is often said to trigger a "Slender sickness"; a rapid onset of paranoia, nightmares and delusions accompanied by nosebleeds.

Early stories featured him targeting children or young adults. Some featured young adults driven insane or to act on his behalf, while others did not, and others claim that investigating the Slender Man will draw his attention. The web series Marble Hornets established the idea of proxies (humans who fall under the Slender Man's influence) though initially they were simply violently insane, rather than puppets of the Slender Man. Marble Hornets also introduced the idea that the Slender Man could interfere with video and audio recordings, as well as the "Slender Man symbol", which became a common trope of Slender fiction. Graphic violence and body horror are uncommon in the Slender Man mythos, with many narratives choosing to leave the fate of his victims obscure.

It is important to note that few of the retellings identify exactly what kind of monster the Slender Man might be, and what his specific intentions are- these points all remain mysteriously and usefully vague.

On May 31, 2014, two 12-year-old girls in Waukesha, Wisconsin allegedly held down and stabbed a 12-year-old classmate 19 times. When questioned later by authorities, they reportedly claimed that they wished to commit a murder as a first step to becoming proxies for the Slender Man, having read about it online. They also state that they were afraid that Slender Man would kill their families if they did not commit the murder. The victim was able to crawl from the woods, where she had been left, to reach a roadside. A passing cyclist intervened, and the victim survived the attack. Both attackers have been diagnosed with mental illnesses but have also been charged as adults and are each facing up to 65 years in prison. One of the girls reportedly said Slender Man watches her, can read minds, and could teleport. Experts testified in court she also said she conversed with Lord Voldemort and one of the Teenage Mutant Ninja Turtles. On August 1, 2014, she was found incompetent to stand trial and her prosecution was suspended until her condition improved. On November 12, 2014, a doctor judged that her condition had improved enough for her to stand trial and on December 19, 2014, the judge ruled that both girls were competent to stand trial. In August 2015, the presiding judge ruled that the girls would be tried as adults.

They are scheduled to be tried separately in September and October 2017. On August 21, 2017, one of the girls pleaded guilty to attempted second-degree homicide.

In a statement to the media, Eric Knudsen said, "I am deeply saddened by the tragedy in Wisconsin and my heart goes out to the families of those affected by this terrible act." He stated he would not be giving interviews on the matter.

After hearing the story, an unidentified woman from Cincinnati, Ohio told a WLWT TV reporter her 13-year-old daughter had attacked her with a knife, and had written macabre fiction, some involving the Slender Man, who the mother said motivated the attack.

On September 4, 2014, a 14-year-old girl in Port Richey, Florida, allegedly set her family's house on fire while her mother and nine-year-old brother were inside. Police reported that the teenager had been reading online stories about Slender Man as well as Atsushi Okubo's manga Soul Eater.

Eddie Daniels of the Pasco Sheriff's Office said the girl "had visited the website that contains a lot of the Slender Man information and stories.
It would be safe to say there is a connection to that."

During an early 2015 epidemic of suicide attempts by young people ages 12 to 24 on the Pine Ridge Indian Reservation, Slender Man was cited as an influence; the Oglala Sioux
tribe president noted that many Native Americans traditionally believe in a "suicide spirit" similar to the Slender Man.