The jurors in Essex Superior Court on Tuesday also found Chism guilty of aggravated rape and armed robbery. He was found not guilty on a second count of aggravated rape.
Chism was charged as an adult and faces life in prison with the possibility of parole between 15 and 25 years, according to Carrie Kimball Monahan from the Essex District Attorney's Press Office.
He showed no obvious emotion as the verdict was being read. A status date in the case is set for December 22.
Chism, now 16, was charged with killing Ritzer, 24, with a box cutter in a bathroom at Danvers High School on October 22, 2013.
Ritzer's body was found a day later in woods near the school.
"This guilty verdict, while the beginning of justice for Colleen, is certainly no cause for celebration as there can never be true justice for the crime committed. There remains a tremendous and painful absence in our lives -- one that, sadly, can never be replaced," said Ritzer's father, Tom.
"Colleen never gave up and neither will we. We will not allow Colleen's death to define how she's remembered," he told reporters.
During closing arguments Monday, defense attorney Denise Regan recounted the testimony of psychiatrists, arguing that Chism has suffered from a psychotic disorder since a young age and that he responded to voices and hallucinations at the time of Ritzer's killing.
"When Philip Chism followed Ms. Ritzer into that bathroom, he was not himself," Regan said before the jury.
"He didn't choose to do this."
The defense conceded that Chism killed Ritzer but presented an insanity defense last week. The court had earlier found Chism competent to stand trial.
Prosecutor Kate MacDougall argued that Chism did not suffer from mental illness and that surveillance video
from inside the school showed Ritzer's killing was planned.
"The only still image that matters in this case is the image of Colleen Ritzer in the woods," MacDougall said.
"There is not one single person in this courtroom who wants to believe that a 14-year-old-boy could have done this and not be crazy. But doing something so awful does not make you crazy."