South African Judge Thokozile Masipa acquitted Pistorius of murder, but found he acted "negligently" in killing the blonde law graduate and fashion model by firing at her through a bathroom door, claiming he mistook her for an intruder.
The 27-year-old athlete -- whose journey from disabled infant to Olympic sprinter inspired the world -- will hear his punishment when sentencing begins on October 13.
His trial had heard that in the early hours of Valentine's Day last year Pistorius fired four hollow point pistol rounds into a locked toilet door, hitting Steenkamp and causing her head to "explode" and "amputating" her arm.
Pistorius stared straight ahead as the conviction was read, showing little emotion.
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But from the courtroom gallery there were sniffles and shallow breaths as friends and family of 29-year-old Steenkamp wept.
Steenkamp's father Barry looked at Pistorius and ran his hand over his head while her mother June pursed her lips and shook her head.
"I just don't feel that this is the right (verdict)," June Steenkamp told America's NBC News after the final judgement was handed down in Pretoria.
The mother said her daughter had died a "horrible, painful, terrible death".
"And I can't believe that they believe that it was an accident," she said.
Pistorius was escorted by more than a dozen policemen wearing bullet proof vests out of the courtroom and into a heaving crowd outside.
Culpable homicide carries no mandatory sentence under South African law.
But Masipa -- who has a reputation for handing out stiff sentences -- could decide to fine Pistorius or put him behind bars for more than a decade.
"It all comes down to how she feels, how bad the mistake was," said Johannesburg lawyer David Dadic. "It's a very serious negligence crime.