The collection of what the website said was of "classified or otherwise restricted files" was termed the "Detainee Policies," ZDNet.com reported.
The documents date back to the administration of George W. Bush.
A 2002 document is said to be the "standard operating procedure" manual for Camp Delta at the U.S. military base in Guantanamo Bay, Cuba.
The document is of "significant historical importance," wrote WikiLeaks founder Julian Assange on the website, because the prison "has become the symbol for systematized human rights abuse in the West with good reason."
Documents yet to be released will show the United States has a systematic program of maintaining unaccountability for the way its military prisoners are treated, the site said.
A 2005 document will allegedly describe the "disappearing" of detainees into the custody of other U.S. government agencies while one from 2008 is purported to show how interrogation documentation is kept to a minimum.
It is not known if the files come from the same documents allegedly leaked by Bradley Manning.