On May 31, 2014, in Waukesha, Wisconsin, United States, Anissa E. Weier (born November 10, 2001) and Morgan E. Geyser (born May 16, 2002), lured their friend Payton Leutner into a wooded area of a local park and stabbed her 19 times to appease the fictional supernatural character Slender Man and prove that he was real. All three girls were 12 years old at the time. Leutner survived the attack after being found by a cyclist, later stating the attack inspired her to pursue a career in medicine.

During their interrogations, Geyser was described as displaying no empathy and Weier was described as displaying guilt for stabbing Leutner, although both stated that the attack was necessary. In 2017, Weier and Geyser were both found not guilty by mental disease or defect and committed to mental health institutions. Weier received a sentence of 25 years to life and Geyser was sentenced to 40 years to life. Weier was released in 2021 to live with her father. Geyser is due to be released in 2025 to live in a group home and will be under supervision until 2058.

The stabbing generated extensive public debate about the role of the Internet in society and its effect on children, and a moral panic occurred over the Slender Man which irreversibly altered the Slender Man legend and the online community surrounding it. The stabbing inspired the documentary Beware the Slenderman, the movie Mercy Black and several books.