Judge Thokozile Masipa who handed down the verdict said a long-term sentence would not serve justice.
“I am of the view a long-term prison sentence will not serve justice in this matter,” she said. “He is not likely to re-offend.”
Steenkamp was shot by the ‘Blade Runner’ at his home in Pretoria, South Africa on Valentine’s Day three years ago.
Former Paralympic athlete Pistorius, 29, claimed he thought his girlfriend was an intruder and the shooting was a tragic mistake.
The sprinter was initially convicted of manslaughter before the verdict was upgraded to murder late last year.
The mandatory sentence for murder is 15 years.
The Supreme Court of Appeal ruled in December last year that he was guilty of murder, citing "errors of law" in the first verdict.
Watch footage from inside the Pistorius trial. Post continues after video...
Judge Masipa who handed down the sentence for murder also presided over the manslaughter charge. She faced criticism for the initial ruling but maintained her judicial role in the second sentencing.
There was great speculation around whether Judge Masipa would be lenient in the sentencing considering she initially found Pistorius guilty of the lesser charge.
Pistorius admitted to ITV that he could still "smell the blood" from the night of her murder.
"I did take Reeva's life and I have to live with that," he said.
"I can smell the blood. I can feel the warmness of it on my hands. And to know that that's your fault, that that's what you've done."
Pistorius said the moments before he shot her were fueled by adrenaline and fear.
"I don't have time to get my legs back on, these people are definitely in the house already, if they come at me quick enough and they grab the gun or if I hesitate, they can use the gun on, on, on either of us."
The verdict released today comes three years after Steenkamp was killed.