​Police officers smashed an innocent autistic teen’s head into ​a concrete sidewalk and punched him in the face as he screamed to his family for help — then released him an hour later with no explanation for their violent conduct, a ​new ​federal lawsuit claims.

Troy Canales, 18, was standing in front of his Bronx home in November 2014 when two cops drove up in a police car and demanded to know what he was doing, according to the Manhattan federal court lawsuit.

“[Canales] was extremely scared, but told the officers that he was just ‘chilling’ and was not doing anything,” the suit states.

“[The officers] each grabbed plaintiff’s arms and forcefully threw him down on the sidewalk, smashing his head against the concrete. [The officers] kneed plaintiff in the back and punched him in the face as he screamed to his family for help.”

Canales’ mom and brother came out of their home and told the cops that he was autistic, but the cops ignored them and took him to the precinct, according to the suit.

Canales — who was 17 at the time — was held for an hour until his mom spoke to the precinct’s commanding officer. The cops wouldn’t explain why Canales was assaulted and arrested except to say that one officer “feared for his life” when he spoke to Canales on the sidewalk, according to the lawsuit.

The suit also claims the NYPD doesn’t give its officers sufficient guidelines for how to deal with autistic people. It seeks compensatory and punitive damages.

​A city Law Department spokesman said the suit is under review.