The jury at London's Isleworth Crown Court rejected prosecution claims that sisters Elisabetta and Francesca Grillo had used credit cards loaned to them by Lawson and Charles Saatchi for household expenses to run up unauthorized charges of 685,000 pounds (more than $1 million) on luxury clothes, designer handbags and high-end hotel rooms.
Defense lawyers for the Grillos - sisters from Calabria in southern Italy - had claimed that Lawson approved their high spending partly in exchange for their silence about her drug use.
Lawson said Friday that she was "disappointed but unsurprised" by the verdict.
"Over the three week trial, the jury was faced with a ridiculous sideshow of false allegations about drug use which made focus on the actual criminal trial impossible," she said in a statement.
In court, the Grillos claimed to have seen signs of drug use around the house - including bags of white powder and rolled-up banknotes - although neither said they had seen Lawson taking drugs.
The allegations shifted the spotlight from the high-spending sisters squarely onto Lawson and Saatchi, who divorced in July after he was photographed grabbing her neck outside a London restaurant.
Lawson accused her ex-husband, a multimillionaire art collector, of spreading false rumors about her frequently using drugs after the damning images surfaced.
"I have been put on trial here ... and in the world's press," Lawson said when she appeared as a prosecution witness at the trial.
On Friday, Lawson said her "experience as a witness was deeply disturbing."
"I did my civic duty, only to be maliciously vilified without the right to respond," she said.
Lawson admitted in court to taking cocaine a handful of times and smoking marijuana occasionally, but strongly denied she was a regular or frequent drug user. She blamed her use of cocaine in 2010 on "intimate terrorism" carried out by Saatchi and her feelings of isolation and fear in her unhappy marriage.
Scotland Yard said Friday it would not investigate allegations that the celebrity cook took cocaine.
The Grillos - 41-year-old Elisabetta or 35-year-old Francesca - were not in court when the verdict was read. Elisabetta Grillo collapsed in court Thursday and was taken to hospital with an anxiety attack. The sisters were allowed to wait in another room at the courthouse for Friday's verdict.
Francesca Grillo declared in Italian "There is a God!" after learning that she had been found not guilty.
Elisabetta Grillo's lawyer, Anthony Metzer, said his client was "relieved" and "crying her eyes out."
The sisters' acquittal could be a blow for Lawson, author of "How to Be A Domestic Goddess" and host of foodie TV shows including "Nigella Bites" and ABC television's cooking program "The Taste."
But Jasmine Montgomery, chief executive of the branding consultancy Seven Brands, said drug allegations would not fatally damage "Brand Nigella" or drive away fans drawn to her warm, sexy and slightly saucy TV persona.
"If you are a sensual, fun, appealing, warm, vivacious, beautiful woman, and it turns out that you are a bit of a partyer, or live slightly on the wild side - I don't think that pulls the wool out from under her," Montgomery said.
"Finding out she took cocaine is not as damaging as if we found out her brownies were made by Mr. Kipling (a supermarket pastry manufacturer)," she added.