The hearing may be the final chapter of his widely watched trial over the fatal 2013 Valentine's Day shooting. Pistorius faces up to 15 years in prison.
Lawyers for the double-amputee sprinter, nicknamed "Blade Runner" for his prosthetic legs, argued that Pistorius was too mentally unwell to serve more jail time.
The only witness called Monday was Jonathan Scholtz, a clinical psychologist, who repeatedly described the athlete as "a broken" man." He explained Pistorius was not able to testify in his own defense due to anxiety and depression following his dramatic fall from grace.
In 2012, Pistorius became the first amputee to compete in the Olympics. But this glory was short lived as he fatally shot Steenkamp through a bathroom door at their Pretoria home, saying he had mistaken her for an intruder.
Scholtz said in his testimony that Pistorius was still on medication for post-traumatic stress disorder and depression. If Pistorius were a regular patient, he would recommend hospitalization, Scholtz said.
But state prosecutor Gerrie Nel questioned why Pistorius had agreed to a television interview in 10 days' time if he was so mentally unfit. He also questioned the psychologist's claim that Pistorius truly felt remorse, saying that the athlete likely just felt sorry for himself.
A court had initially convicted Pistorius of culpable homicide, but the Supreme Court in December overturned the conviction and handed him the more serious verdict of murder. Pistorius appeared shaken throughout the trial, looking gaunt and dazed and throwing his head into his hands at times.
Scholtz said that Pistorius "takes some comfort" in the knowledge that Steenkamp is with God, and that the athlete, a known gun collector, had sold all his weapons and had sworn off firearms.
Pistorius arrived in the Pretoria court in a dark suit and tie flanked by a large police escort. His relatives sat on one side of the courtroom while the family of Steenkamp sat on the other.
There was some debate about the victim's mother, June Steenkamp, and her statement that she had forgiven Pistorius. Scholz and prosecutor Gerrie Nel agreed that she had done that for her own sake, not for the man who killed her daughter.
Pistorius will learn his fate by Friday.
Here's what else you need to know:
But hasn't he been sentenced already?
Yes, after a nearly 50-day trial stretched over seven months, Pistorius was convicted of culpable homicide (much like manslaughter) in September 2014. Judge Thokozile Masipa ruled the sprinter had acted negligently when he shot Steenkamp four times through a locked bathroom door, but that he didn't do it intentionally. The Supreme Court of Appeal later changed that conviction to murder.
Why did they change the verdict?
The appeals court ruled that the identity of whoever was behind the bathroom door was irrelevant. Pistorius should have foreseen that his action would kill that person, but he went ahead anyway. The key legal principle is known as dolus eventualis.
Is he in prison?
No, Pistorius is at his uncle's mansion under house arrest. Pistorius spent about a year in a private cell in the hospital wing of a maximum-security prison. He should have gotten out after 10 months, or a sixth of his sentence, but a South African minister intervened. Pistorius could be going straight back to the cell after the sentencing hearing.
What to look out for at this sentencing
It will be the first time the public gets a good look at Pistorius since the first sentencing hearing. The defense has tried to paint a picture of a "broken man" and will likely focus on the media coverage and public treatment of Pistorius as one of their arguments. On the prosecution side, we may see a family member or close friend of Steenkamp brought as a witness to offer emotional testimony.
What sort of time will Pistorius do?
The sentence for murder in South Africa is 15 years, but the defense will argue the judge has some discretion to reduce the jail time and include time already served. Nel, the prosecutor, is unlikely to go for the lower end of the sentencing spectrum. Trial judge Masipa will be handing down the sentence.