Morgan Geyser and Anissa Weier are charged with attempted first-degree intentional murder over the attack.
The ruling means they may face decades in jail. Their lawyers failed to get the case heard by a juvenile court.
If tried as juveniles, the pair only face a five-year term.
The victim, 12 at the time, was stabbed 19 times but survived.
She was found by a cyclist crawling from the woods where she was attacked with stab wounds to her arms, legs and torso, after the attack in May 2014.
Wisconsin Judge Michael Bohren decided the pair, now both 13, should remain in adult court after four days of hearings in May and June.
They told investigators they had stabbed their victim in "dedication" to Slender Man, a fictional horror website character.
They spoke of their desire to become the paranormal figure's "proxies" by killing to demonstrate their loyalty, police said.
Following the attack, both suspects were found walking near a local highway, and a knife was found in one of their backpacks, police said.
They said the girls planned to stab the classmate during a sleepover but instead decided to commit the crime the next morning in a nearby park.
After the murder, they reportedly intended to walk to Slender Man's mansion, which they believed was situated in Wisconsin's Nicolet National Forest.
The victim has recovered and since returned to school.
Two 13-year-old girls accused of stabbing a classmate to please the online horror character Slender Man will be tried in an adult court, a US judge has ruled.