JOHANNESBURG — South Africa’s top appeals court ruled on Thursday that Oscar Pistorius, the Olympic star known as the Blade Runner, was guilty of murder in the 2013 killing of his girlfriend, Reeva Steenkamp, overturning a lower court’s conviction on the lesser charge of manslaughter.

Mr. Pistorius has already served about a year in prison and is now under house arrest for the previous conviction; a murder conviction carries a minimum sentence of 15 years in prison.

The appeals court sided with state prosecutors on central points, saying that the manslaughter conviction, technically called culpable homicide, had been based on a misinterpretation of laws and an erroneous dismissal of circumstantial evidence.

The court said that Mr. Pistorius, who has said repeatedly that he accidentally killed his girlfriend when he believed his home had been broken into, should have foreseen that his actions would cause the death of a person.