In a statement, the Pistorius family said neither it nor the Steenkamps were consulted about the film, "Blade Runner Killer", which is set to premiere on the Lifetime network in the US on November 11.
The athlete's family criticised the project for not being a "true reflection" of the shooting nor of Pistorius's marathon trial which gripped the world.
"The 'film' is a gross misrepresentation of the truth," the family said in a statement. "We will be taking legal action."
The double-amputee sprinter shot Steenkamp four times with high-calibre bullets through a locked bathroom door.
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He maintained that he mistook the model for an intruder.
The family statement, issued by Oscar's brother Carl Pistorius, dismissed the film as a biased "representation of what the prosecution tried to portray."
On Monday, the Steenkamps also distanced themselves from the film, saying they were "horrified and upset" at reports that it claimed to tell the story from the perspective of Reeva and her mother June.
In 2016, an appeals court upgraded Pistorius's manslaughter conviction to murder.
South African state prosectors are pushing for a longer sentence, with the appeals court due to hear the case on November 3.