The Daily Mail suggested Mrs Trump may have worked as a part-time escort in New York, and met husband Donald Trump, who is now running for the White House, earlier than previously reported.
Blogger Webster Tarpley wrote that Mrs Trump feared her past becoming public.
The claims were "outright lying", lawyer Charles Harder said.
"These defendants made several statements about Mrs Trump that are 100% false and tremendously damaging to her personal and professional reputation," Mr Harder said in a statement.