Prosecutors said they were disappointed by the ruling but would decide on whether to appeal only after sentencing.
A conviction of culpable homicide, or negligent killing, can bring a maximum prison sentence of 15 years.
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 proved beyond a reasonable doubt that Pistorius intended to kill Steenkamp.
Pistorius' family thanked Masipa for the lack of a murder charge in a statement released Friday morning.
"It is a big burden off our shoulders and Oscar," the statement read. "We always knew the facts of the matter. We never had any doubt in Oscar's version of this tragic incident."
Still, the family noted there are "no victors in this."
"We as a family remain deeply affected by the devastating tragedy," they wrote. "It won't bring Reeva back, but our hearts still go out for her family and friends."
Masipa said Pistorius' sentencing hearing would begin Oct. 13. Both sides will be able to call witnesses to argue ahead of the judge's ruling on if, or for how long, Pistorius is sent to prison for negligently killing Steenkamp.
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 hometown. "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 are not going to get any closure because this guy is basically getting away with murder, in my opinion."
However, Shrina Padayachy, also in Port Elizabeth, called the judge's verdict "fair and just because it's the prosecution that must prove beyond a reasonable doubt, and from what I gather and seen, they haven't."
The red-robed judge ordered Pistorius, 27, to stand before she delivered the formal verdict on the multiple counts against him and said it was a unanimous verdict, meaning she and her two legal assessors agreed on the findings.
Unlike many other times during a trial that began in March, Pistorius showed no emotion as he stood in a dark suit with his hands crossed in front of him. The double-amputee Olympian was hugged by relatives when the judge ordered a recess soon after announcing her verdicts. The judge later ruled that Pistorius be granted bail with the same terms as before.
Some legal analysts said they understood why Pistorius was found not guilty of premeditated murder but were surprised that the runner was not convicted of murder.
"We believe there is sufficient and credible evidence to secure a conviction" on a murder charge, said Nathi Mncube, spokesman for the National Prosecuting Authority. He added that any decision to appeal the judge's ruling would come after the case is "concluded" with sentencing.
The sentence for a culpable homicide conviction is at the judge's discretion, and it can range from a suspended sentence and a fine to up to 15 years in prison.
Once he serves whatever punishment is handed down, Pistorius will be able to compete again in the Paralympics, the International Paralympic Committee said Friday afternoon.
After the verdict, Pistorius sat with his sister, Aimee, on the wooden bench where he has spent most of his six-month murder trial. She put an arm around his shoulders and spoke to him.
For the first time in the trial, Pistorius left by going down the stairs that led to the cells in the courthouse. That's because Pistorius' bail expired after his conviction, and the judge was considering whether to regrant him bail. One of Pistorius' lawyers, Brian Webber, said Pistorius was taken to a "holding position."
Defense lawyer Barry Roux told the judge that Pistorius should stay free on bail until sentencing because he had complied with bail conditions imposed on him after he killed Steenkamp. Chief prosecutor Gerrie Nel, however, said Pistorius now had more reason to flee because he had probably hoped for an acquittal during the trial.
"Now he knows for a fact that, 'I've been convicted of a serious offense and imprisonment is probable,"' Nel said.
Members of Steenkamp's family, including her parents, were in court to hear the verdict in the 29-year-old model's killing.
The judge ruled that the athlete was guilty of unlawfully firing a gun in a public place when a friend's pistol he was handling discharged under a table in a restaurant in Johannesburg in early 2013 -- weeks before Steenkamp's killing.
He was acquitted on two other weapons charges, including a count of firing a gun in public and a count of illegal possession of ammunition in the Pretoria home where he killed Steenkamp.
He said he mistook her for an intruder, while the prosecution said he killed her intentionally after an argument.
On Friday, armed security officers stood at each of the three entrances to the courtroom, while others stood near the judge as she explained her verdicts from her dais overlooking the court. There were also paramedics in the courtroom.
The Associated Press contributed to this report.
Pistorius Found Guilty Of Culpable Homicide
ABC News' Matt Gutman discusses the judge finding Oscar Pistorius guilty of culpable homicide in the shooting death of girlfriend Reeva Steenkamp.