Cliff Sims filed a lawsuit Monday in federal court in Washington.
Trump's campaign accused Sims last month of violating a nondisclosure agreement by publishing his book, "Team of Vipers."
Trump has dismissed Sims as a "low level staffer." He said the book was "based on made up stories and fiction."
Sims' lawsuit charges the government is using a private surrogate to "serve as an illegitimate cutout" and impose liability through the nondisclosure agreement. Sims argues he learned the information only through his time as a federal employee.
The White House didn't immediately comment.