Prosecutors said they were disappointed by the ruling but would decide on whether to appeal only after sentencing.
Judge Thokozile Masipa said there was not enough evidence to support the contention that Pistorius knew Steenkamp was behind a locked toilet door in his home when he shot through the door in the predawn hours of Valentine's Day last year.
Masipa said prosecutors had not proven beyond a reasonable doubt that Pistorius intended to kill Steenkamp.
Pistorius said he mistook Steenkamp for an intruder when he shot her, while the prosecution said he killed her intentionally after an argument.
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The killing by a national sports hero stunned South Africans, and reactions on the verdict were mixed.
"Well I think the verdict is shocking to say the least," said Leonard Gray in Port Elizabeth, Steenkamp's home town. "I think there is clear evidence and indications that he committed murder in this case. ... I feel sorry for Reeva Steenkamp's family because they're not going to get any closure because this guy is basically getting away with murder in my opinion."
Free again after the judge extended his bail, the double-amputee Olympian left the courtroom surrounded by police officers. Pistorius must return on Oct. 13 when his sentencing hearing will start, Judge Masipa said. Pistorius didn't comment.
The red-robed judge earlier ordered Pistorius, 27, to stand before she delivered the formal verdicts on the multiple counts against him, and said they were unanimous verdicts, meaning she and her two legal assessors agreed on the findings.