The defendant, Michael McCarthy, was dating the victim's mother, Rachelle Bond, at the time of the girl's death. Her body was found in June 2015.
McCarthy faces a mandatory sentence of life in prison with the possibility of parole in 15 years, the Boston Globe reported.
The Associated Press reported that McCarthy's lawyer told jurors that Bond is a “monster” who killed her own daughter. McCarthy’s lawyer said in closing arguments that Bond created a “web of lies” and placed the blame on McCarthy, according to the AP.
McCarthy's recorded interview with police was played in court, in which he denies killing Bella and says Bond put the blame on him.
Bond testified that on the night Bella died, McCarthy went to the 2-year-old’s room to tell Bella to lay down. Five minutes later she returned to the room, where she witnessed McCarthy punching Bella in the stomach, she continued.
"I saw her bounce off the bed" and then fall back down, Bond told the jury.
She said Bella's "head was swollen and gray” and she tried to do CPR.
She testified that she picked Bella up to leave but McCarthy grabbed her by the throat with both hands and said he'd kill her.
Bond told the court she blacked out. When she woke up, she was on the couch in the living room.
She testified that she told McCarthy he had killed Bella. McCarthy replied, "It was her time to die, she was a demon," Bond said.
In June 2015 the girl's unidentified body, a zebra-print blanket and polka dot leggings were found on Deer Island, about eight miles east of Boston's Logan Airport.
Computer composites used to identify the body were viewed by millions and the mystery surrounding the unknown victim captivated the country.
In September 2015 Bella was finally identified, and McCarthy and Bond were arrested.
Bond, who was charged as an accessory, pleaded guilty earlier this year in exchange for her testimony against McCarthy, ABC affiliate WCVB reported in February. She is currently incarcerated. After the trial, she will be released from jail and will serve probation for two years, according to the plea agreement.
McCarthy will be sentenced Wednesday to life in prison with the possibility of parole in at least 15 years.
Colonel Richard McKeon, superintendent of the Massachusetts State Police, said in a statement, “This not a joyous day, but it is a necessary one. The verdict cannot bring Bella back, and cannot change the fact that she was failed – colossally and tragically – by the adults in her life. But the verdict is right, and justice has been rightly served upon the person who took this beautiful child’s life.”
ABC News' Aaron Katersky contributed to this report.