A mentally disabled girl was ruthlessly beaten by a group of teens in Chicago as bystanders cruelly laughed and recorded the assault, disturbing video shows.

A 42-second clip posted to social media on Monday shows the girl, 15, walking with a group of several other girls and at least one boy when one of the girls suddenly punches her from behind.

A witness who was already recording the teens walking then immediately starts laughing as the other girls — as many as five at one point — join the unprovoked attack.

“They beating her,” one teen says while chuckling.

The graphic footage, which has been viewed more than 5.1 million times, ends as her screams for help and someone appears to grab her hand to get her back on her feet.

One of her friends later posted the footage in hopes of getting “justice” for the girl, whose mental disability has progressed since her mother died, according to the now-viral post.

“She didn’t deserve any of this,” her friend wrote.

The video, meanwhile, has caught the attention of Chicago police, including spokesman Anthony Guglielmi, who characterized it as “beyond disturbing” to watch.

“This young girl deserves far better,” Guglielmi wrote late Tuesday. “This clip has been forwarded to the Chicago Police Special Victims Unit and a criminal investigation has been launched.”

The teen was reported missing Tuesday, but has since been safely located and reunited with her family. An investigation into the footage is ongoing, police spokeswoman Kellie Bartoli told The Post early Wednesday.

The exact location of the assault is unclear, but her friend who posted the footage said she was attacked as the group walked her to a nearby Red Line train station. One of the girls said they jumped her because she was talking to police, video shows.