"I am kidnapped," the 30-year-old captive wrote.
"For the love of God, call the police. I am in a house. Make an anonymous report. Save me, please. I am a woman with children. I'm very afraid. I don't know where I am."
The woman had been snatched from her own doorway by four men who demanded a ransom of 5 million reais ($2.7 million) for her safe return, according to police.
Taking advantage of a moment last Saturday when none of her captors was watching, the woman managed to write the cry for help and slip it out of window.
Someone picked up the note and alerted police, who mounted a rescue and arrested one of the captors, Eduardo Luis da Silva, who was carrying a handgun. Investigators suspect as many as five other people may have been involved in the kidnapping.
The freed woman was said to be in good health.