The measures, which take effect immediately, bar credentialed reporters from most of the Department of Defence headquarters in Arlington, Virginia, unless they have an official approval and escort.
"While the Department remains committed to transparency, the Department is equally obligated to protect CSNI (classified intelligence) and sensitive information - the unauthorised disclosure of which could put the lives of US Service members in danger," Hegseth said in a memorandum.
He called the protection of classified national intelligence information and operational security "an unwavering imperative for the Department."
The Pentagon Press Association, a membership organisation representing the interests of the press corps covering the US military, said the new rules appeared to be a "direct attack on the freedom of the press."
"The decision is purportedly based on concerns about operational security. But the Pentagon Press Corps has had access to non-secured, unclassified spaces in the Pentagon for decades, under Republican and Democratic administrations, including in the aftermath of the September 11, 2001 attacks, without any concern about OP-SEC from DoD leadership," the statement said.
The Pentagon did not immediately respond to a Reuters request for comment on the press association's statement.
Since President Donald Trump returned to office in January, the Pentagon has launched an investigation into leaks that resulted in three officials being placed on administrative leave.
It has also required legacy media organisations, including the New York Times, the Washington Post, CNN and NBC News, to vacate their Pentagon office spaces in a new rotation system that brought in others, including outlets generally friendly to the Trump administration such as the New York Post, Breitbart, the Daily Caller and One America News Network.
The Trump administration says the move is intended to allow other media outlets the opportunity to report as resident members of the press corps.
More broadly, the Trump administration has deployed lie detector tests to investigate unclassified leaks, with some Department of Homeland Security officials being told they could be fired for refusing polygraphs, Reuters reported. The White House says Trump will not tolerate leaks to the media and that federal employees who do so should be held accountable.
Hegseth's order also requires members of the Pentagon press corps to acknowledge a responsibility to protect national intelligence and sensitive information, and says they will be issued with new credentials that more prominently identify them as members of the press.
"We also anticipate a forthcoming announcement of additional security measures and enhanced scrutiny on the issuance of (credentials)," the memo said.
