In a statement after a fourth round of questioning before magistrates, Lagarde said she would return to her work in Washington later in the day and said the decision was "without basis."
She and her former chief of staff face questions of negligence in connection with their role in a 400-million euro ($531 million) payment to a businessman.
"After three years of proceedings, dozens of hours of questioning, the court found from the evidence that I committed no offense, and the only allegation is that I was not sufficiently vigilant," she said in her statement.
Under French law, the official investigation is equivalent to preliminary charges, meaning there is reason to suspect an infraction. Investigating judges can later drop a case or issue formal charges and send it to trial.
The payment was made to Bernard Tapie in arbitration over a dispute with state-owned bank Credit Lyonnais over the botched sale of sportswear company