The office said Thursday that it dropped the case due to a lack of sufficient proof.
Officials said the former International Monetary Fund chief admitted to sexual aggression against writer Tristane Banon but that it's too late to prosecute for that charge, because it happened in 2003.
The writer said Strauss-Kahn tried to rape her during an interview.
Strauss-Kahn resigned from the IMF's top job earlier this year in the wake of a scandal involving a New York maid.
The Associated Press contributed to this report.