This follows the Pentagon's launch of an investigation into Kelly after he and five other Democrats in Congress published a video statement saying the military should refuse to carry out "illegal orders."
"This administration is pitting our military and intelligence officials against American citizens," they said, among other things.
According to a statement from the Pentagon in November, Kelly's comments in the video impinge on "the loyalty, morale, or order and discipline of the armed forces."
Kelly himself, however, says that the investigation violates the US Constitution and his right to freedom of speech. In addition to Hegseth and the Pentagon, the lawsuit names Navy Secretary John Phelan and the Navy Department.
"I was just quoting the law: that military personnel must refuse to carry out illegal orders. In some ways, quoting the law is now illegal, according to Pete Hegseth," Kelly said in the Senate on Monday.
The Pentagon is "aware" of the lawsuit but "does not comment on ongoing litigation," the department wrote in a comment to AFP.
The senator served as a fighter pilot and later became an astronaut. When he retired from the military, he had reached the rank of commander.
