Judge Thokozile Masipa said the athlete acted “negligently” when he fired shots through a toilet door, but in the “belief that there was an intruder”.
She said the state had failed to prove he intended to kill Reeva Steenkamp.
The judge allowed Pistorius to remain on bail ahead of sentencing which is to take place on 13 October.
Arnold Pistorius, the athlete’s uncle, said the family was “deeply grateful” to the judge for finding him not guilty of murder and that a “big burden” had been lifted.
“There are no victors in this,” he added. “We as a family remain deeply affected by the devastating, tragic event… It won’t bring Reeva back but our hearts still go out for her family and friends.”
South Africa’s prosecuting authority said it was “disappointed” Pistorius was not convicted of murder but said it would wait until after sentencing to decide whether to appeal.
Speaking outside the court, Nathi Mncube, for the National Prosecuting Authority, he said: “We respect the judgment that has been delivered.
“We believed in this instance there was enough evidence to secure a conviction under pre-meditated murder.
“Of course we are disappointed. We are disappointed we did not secure a conviction under pre-meditated murder and also there was acquittal on the other two (gun) charges.
“The matter has not been concluded yet, we are still waiting for a sentence to be imposed.”
Masipa’s decision also sparked anger outside the court, particularly among those campaigning for women’s rights in a country with high levels of violent crime against women and children.
The verdict leaves the disgraced sprinter facing up to 15 years in jail, although the judge could suspend the sentence or only impose a fine.
The athlete was also found guilty on a charge of negligently handling a firearm that went off in a restaurant.
Oscar Pistorius was a picture of composure as the judge announced he was guilty of culpable homicide, a lesser count of murder in South Africa.
The athlete, who became emotional on Thursday after being described as an “evasive witness”, seemed to have been expecting this verdict. The judge had already spoken of his negligence and use of “excessive force” when he fired through the door.
Some say this verdict is the best outcome he could have hoped for.
Many South Africans have questioned the judge’s decision to acquit him of murder and argue that the prosecution may have grounds to appeal.
For now, the Pistorius family seem relieved. But in the benches opposite them, friends and relatives of model Reeva Steenkamp wept, while others, including her mother June, seemed shell-shocked.
The double amputee had denied murdering Ms Steenkamp after a row on Valentine’s Day last year, saying he shot her by mistake.
With this the judge agreed, saying “it cannot be said that the accused did not entertain a genuine belief that there was an intruder”.
But she said: “The accused knew there was a person behind the toilet door; he chose to use a firearm.
“I am of the view the accused acted too hastily and used too much force. It is clear his conduct is negligent.”
Pistorius was convicted on an unrelated firearms charge – negligently handling a firearm that went off in a crowded restaurant.
He was acquitted of another charge of firing a gun in public, through the sunroof of a car, and of a charge of illegal possession of ammunition in the home where he killed Ms Steenkamp.