Tristane Banon's lawyer David Koubbi said the lawsuit will be filed in Paris on Tuesday.
He said the complaint will relate to an incident that took place when Banon went to interview Strauss-Kahn in an apartment in Paris.
Banon was 22 at the time. She has already publicly discussed the incident, saying Strauss-Kahn acted like a "chimpanzee on heat".
Koubbi said Banon had been dissuaded at the time from filing charges by her mother, a regional councilor in Strauss-Kahn's Socialist party.
Strauss-Kahn was arrested in New York in May on charges that he tried to sexually assault a hotel maid. The former head of the IMF, who denied wrongdoing, was released without bail last week after questions emerged about the maid's credibility.
Koubbi had said in the past that they would not file a lawsuit until the American trial was finished. He said on Monday that they had decided to move forward now instead of waiting.
The IMF will not shy away from commenting on controversial currency issues in its new role supporting the G20 accord on a framework for balanced global growth agreed in Pittsburgh
The scale of the economic slowdown in Asia was starkly underlined yesterday when the International Monetary Fund virtually halved its 2009 growth forecast for the region. The IMF’