He was also cleared of second degree murder but faces a remaining charge of culpable homicide – the South African equivalent of manslaughter – as well as firearms offences.
Ms Steenkamp, 30, was killed at double-amputee Pistorius’ home on February 14, 2013.
Pistorius, 27, said he thought his girlfriend was in his bedroom and that intruders were in his bathroom when he fired four shots.
Delivering her verdicts, Judge Thokozile Masipa said: “The accused believed his life was in danger. The accused cannot be found guilty of murder.”
The Olympian sat sobbing with his head bowed in court as Judge Masipa read the verdicts. He sat alone on a bench seat in the hushed court room.
Although Masipa described the athlete known as the Blade Runner as a “very poor” and “evasive” witness, she said this did not mean the track star was necessary guilty in a case that she said was based entirely on circumstantial evidence.
The judge said she believed accounts that an emotional Pistorius gave to police at the scene in the moments after the shooting.
She said: ‘”Counsel for the defence correctly argued that it was highly improbable that the accused would have made this up so quickly and be consistent in his version, even at the bail application, before he had access to the police docket and before he was privy to the evidence on behalf of the state at the bail application.
“The question is, did the accused foresee the possibility of the resultant death, yet persisted in his deed reckless of whether death ensured or not?
“In the circumstances of this case, the answer has to be no.
“How could the accused reasonably have foreseen that the shot he fired would kill the deceased? Clearly he did not subjectively foresee this as a possibility that he would kill the person behind the door, let alone the deceased – as he thought she was in the bedroom at the time.”