The jury of six men and six women took about five and a half hours to reach its verdict.
The body of 15-year-old Tiffany Gayle was found in a tub in the basement of her family home three years ago. An autopsy would reveal that she had died from blunt-force trauma.
Her father, Federick, and stepmother, Elizabeth, were arrested three days after the teenager was found dead.
On Wednesday, a jury found the couple guilty of first-degree murder.
Defence lawyer Stephen Bernstein said he will appeal the decision.
"We feel there was ample room for reasonable doubt," he said.
But Crown prosecutor Brian McGuire said the evidence spoke clearly and directly to the jury.
"The evidence that she [Tiffany] left behind at the scene — she was the only witness we couldn't call and she was the most powerful witness in the case."
During the trial, the Crown had told jurors that the couple had used a baseball bat and barbecue utensil when inflicting the fatal blows on the teenager.
Jurors also heard about the ongoing tension between Tiffany and her guardians.
The verdict means Elizabeth and Federick Gayle will both be sentenced to life in prison.