The 13 people on trial in the province of Tucuman had faced up to 25 years if convicted on charges that they abducted Maria de los Angeles "Marita" Veron and made her work as a prostitute in 2002.
Veron's mother, Susana Trimarco, launched a one-woman campaign to hunt for her, and rescued hundreds of women from sex slavery along the way.
She has been honoured by the US State Department and the Argentine government, and even nominated for a Nobel Peace Prize, but she has never found her daughter.
Top government officials in Argentina backed Veron's mother in the year-long trial.
Trimarco was honoured with a human rights award from President Cristina Fernandez on Sunday night.
In the end, the three-judge panel delayed for more than four hours on Tuesday night before reading their unanimous verdict.
The courtroom erupted at the news, with the defendants sobbing and spectators shouting expletives.
Politicians for and against Argentina's government tweeted in support of Trimarco.
The judges later explained from the bench that despite the testimony of more than 130 witnesses, including a dozen former sex slaves who described brutal conditions in brothels, there was no physical evidence linking any of the defendants to Marita Veron, and no trace of her whereabouts.
The defendants, who were all accused of participating in organised crime in Argentina's provinces, told the judges as the trial wrapped up that they were the victims of a politically charged process.
"It's absolutely clear that this is an act of corruption," said one of Trimarco's lawyers, Jose D'Antona.
Trimarco was clearly upset but kept her composure as she left the courthouse. Later, she told Argentina's TN channel that she will promote an effort to impeach the three judges.
"We won't stop until these three con men are put on trial. These judges today were an embarrassment for Argentina," Trimarco said.
Defense lawyer Hernan Molina defended the judges, however, saying they had done the right thing.
"There wasn't any evidence," he told reporters at the courthouse. "The judges can't convict innocent people."
In an interview to AP, she recalled how the very first woman she had rescued taught her to be strong.
"She told me not to let them see me cry, because these shameless people who had my daughter would laugh at me, and at my pain," Trimarco recalled.
"Since then I don't cry anymore. I've made myself strong, and when I feel that a tear might drop, I remember these words."
Maria Jesus Rivero, owner of the car service allegedly used to kidnap Veron, told the judges he had already been effectively convicted by Argentine Security Minister Nilda Garre, who had asked the judges to set an example by convicting them all.
"There isn't a single piece of evidence that links me to the people accused here, just the declarations of liars. Marita isn't here, and we don't have anything to do with this," Rivero said.
