The South African sprint runner killed his girlfriend in February 2013 after shooting her four times through a locked toilet door.
The 29-year old is currently under house arrest after spending one year of his original five-year sentence in jail.
His jail term is yet to be decided, but a murder conviction carries a much heftier sentence: a minimum of 15 years in South Africa.
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Pistorius had said he accidentally shot dead Miss Steenkamp after believing there was an intruder in his house. The prosecution argued he killed her after an argument.
Reading the ruling reached by a panel of five judges, Justice Eric Leach called the circumstances surrounding Steenkamp’s death “a human tragedy of Shakespearean proportions.”
The ruling rested over the concept of “dolus eventualis,” or whether Pistorius should have foreseen his actions would lead to death.
“All the shots fired through the door would almost inevitably have struck the person behind it. There would be effectively no place to hide.
He called Pistorius’ testimony about his actions “unacceptable,” “vacillating” and “contradictory.”
In addition Leach stated that although “Pistorius had genuine beliefs that his life was in danger, he should have acted more rationally. He never fired a warning shot, and shot not once but four times.
“The identity of victim is irrelevant to his guilt.”
Pistorius will have to return to court to be re-sentenced, for murder.