The white supremacist group will hold the event on 5 October at the site of the July 1863 Battle of Gettysburg in Pennsylvania.
Park officials said US freedom of speech rules required them to make the site available to the group.
As many as 50,000 soldiers were killed or wounded at the Battle of Gettysburg.
The battlefield, named for the small town nearby, is also known as the site of the Gettysburg Address, a short speech by President Abraham Lincoln that became one of the most celebrated moments of American oratory.
At the dedication of a cemetery there the following November, Lincoln envisioned "a new birth of freedom" for America out of the ashes of the war between the southern slave-holding states and the northern free states.
The Ku Klux Klan, a white supremacist group established in the aftermath of the conflict, is known for its violent past and white-hooded ceremonial costume.
Richard Preston, leader of the Confederate White Knights of the Ku Klux Klan, was quoted by a local newspaper as saying his group was a "white separatist" organisation dedicated to throwing US President Barack Obama, who is black, out of office and to advocating stricter immigration laws.
"This is about America for Americans," Mr Preston said. "The only thing we want is our country back."
The group held a membership rally earlier this month at the Antietam battlefield in Maryland, the site of the deadliest single-day battle of the war.
In a statement, the National Park Service said: "As custodians of land owned by the American people, the National Park Service has a responsibility to make that land available for exercising [free speech] rights."
Park officials added that Gettysburg staff would work "to ensure that park resources and visitors will not be adversely affected by this event".
A Maryland Ku Klux Klan group has been granted a permit to hold a rally at a renowned Civil War battlefield, US authorities have said.