Judge Thokozile Masipa ruled that the state did not prove beyond reasonable doubt that Pistorius intended to kill the South African model at his home in Pretoria on February 14 last year.
However, the double amputee has been found guilty of culpable homicide (manslaughter) and of one firearms charge.
Arnold Pistorius told the media in the High Court: "On behalf of the family, we would really like to show how deep and how grateful we are to Judge Thokozile Masipa who has found Oscar not guilty of murder - that's a big burden off our shoulders.
"It will be highly inappropriate to make any further statements at this stage regarding merit of the case and findings of the court. We respect the fact that the legal process is not over and must always be run in the course of justice.
"Furthermore, a tragic event like this, there are no victors in this. We as a family remain deeply affected by the devastating tragedy. It won't bring Reeva back, but our hearts still go out for her family and friends."
Pistorius, who maintained that he mistook Steenkamp for an intruder when he fired four gunshots through a toilet door at his home, will be sentenced on October 13.