Judge Masipa is yet to rule on whether the athlete is guilty of culpable homicide - manslaughter - over his girlfriend's death.
Video: Oscar Pistorius shot Reeva Steenkamp four times
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Judge Thokozile Masipa has told the Oscar Pistorius trial "there are not enough facts" to support the prosecution case that the athlete intended to kill his girlfriend Reeva Steenkamp.
She told the court there was "no doubt" when Pistorius fired his gun "he acted unlawfully", but is still to rule on whether he is guilty of a lesser crime of culpable homicide - manslaughter in other countries - over the 29-year-old model's death.
Judge Masipa cleared the track star of both premeditated and second-degree murder three hours into delivering her judgment on whether Pistorius will spend years in prison for shooting dead Ms Steenkamp.
She said the charge of premeditated murder relied on "circumstantial evidence" from the State.
"The State has not proved beyond reasonable doubt that the accused is guilty of premeditated murder. There are just not enough facts to support such a finding," she said.
Sky's Alex Crawford, in the courtroom, said: "She very quickly early on dismissed the central pillars of the State's case."
Pistorius was charged with one count of murder, two of illegally discharging a firearm - which is unrelated to the death of Ms Steenkamp - and one of illegally possessing ammunition. He pleaded not guilty to all the charges.
The 27-year-old, his head down and shoulders shaking, was visibly distressed and wept in court as the judge recounted the events of Valentine's Day last year.
The prosecution suggested he stormed down his bedroom corridor after his frightened girlfriend in a jealous rage and deliberately shot her through the toilet door in an angry tantrum on that fateful day.
It had pushed for a verdict of premeditated murder which could carry a sentence of 25 years in prison.
The double-amputee sprinter insisted he was terrified and felt excessively vulnerable due to his disability and shot Miss Steenkamp in a tragic accident believing her to be an intruder.
South Africa has no jury system so Judge Masipa is deciding on the verdicts herself with the help of two assessors.
Proceedings kicked off on Thursday with the judge outlining the facts of the case before the testimonies of each of the 37 witnesses who gave evidence at the six-month-long murder trial.
Some 21 witnesses testified for the prosecution and 16 for the defence.
She said claims by the defence that police contaminated evidence and removed items from the crime scene "paled into insignificance".
The judge also discounted claims by the prosecution that the relationship between Pistorius and Ms Steenkamp was "on the rocks" - and counter claims by the defence that they were a "loving couple".
"In my view none of this evidence from the State and defence proves anything," she said.
"Normal relationships are dynamic and unpredictable most of the time, while human beings are fickle.
"Neither the evidence of a loving relationship, nor a relationship turned sour can assist this court to determine whether the accused had the requisite intention to kill the deceased."
The judge also found discrepancies between witness recollections and phone records, and said she would be relying on the latter.
She said Pistorius was a poor and evasive witness who was initially composed, then "contradicted himself" under cross examination.
Ms Steenkamp's parents - June and Barry Steenkamp - looked on and listened intently to Judge Masipa from a packed public gallery.
More follows...
