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.
He was also found guilty on a charge of negligently handling a firearm that went off in a restaurant.
The BBC’s Andrew Harding, in court, says that following the verdicts, Pistorius sat, rubbed his face and slumped briefly forward.
There was not much emotion in court because everyone knew that verdict was coming, our correspondent adds.
It leaves the disgraced sprinter facing a jail sentence of up to 15 years, although legal experts suggest seven to 10 years is more likely when sentence is passed in a couple of weeks.