Oscar Pistorius has been found guilty of culpable homicide after Judge Thokozile Masipa concluded that the athlete killed Reeva Steenkamp by mistake.
The Olympic athlete is yet to be sentenced for his crimes, but the charge can carry a maximum sentence of 15 years.
The verdict came on Friday morning, after the judge acquitted Pistorius of premeditated murder and common-law murder. She stated that the state had failed in their attempt to prove he plotted to kill Ms Steenkamp.
However, Judge Masipa declared that Pistorius had acted "negligently" when he fired through the toilet door last year, but believes he did so with the "belief that there was an intruder".
The multiple Paralympic gold medal winner was also found guilty of handling a firearm that was fired in a restaurant. That charge could add up to five years to the charge, but could also be settled with a fine.
Pistorius was acquitted for another charge for firing a fun through a sunroof and was also cleared of illegal possession of ammunition.
Pistorius has denied intentionally killing Ms Steenkamp on Valentine's Day in 2013, with the state arguing he had done so with intention after an argument in his home.
The judge concluded: "It cannot be said that the accused did not entertain a genuine belief that there was an intruder.
"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 will now await his sentence where he could serve up to 20 years. However, as he is a first time offender, any prison term is likely to be significantly less.
Lead prosecutor Gerrie Nel has already stated that Pistorius is likely to face a lengthy sentence and has requested the court deny him extended bail.