Michael Monckton, 28, was found guilty of second-degree murder. The jury convicted him in the death of Keagan Davis in January 2010.
Monckton was living with Keagan's mother, Leigh-Ann Worral, in an Oshawa apartment when the toddler died of abdominal trauma and internal bleeding.
The court heard the child also suffered from numerous other injuries.
Keagan had several bruises on his head, back and stomach and several of those injuries were inflicted minutes and hours before death, according testimony from a pathologist.
Monckton testified the child injured himself when he slipped from a chair.
The jury deliberated for more than a day and returned its verdict on Wednesday afternoon.