The FBI searched journalist Hannah Natanson's devices and seized a phone and a Garmin watch at her Virginia home, the Post said.
Natanson covers the administration of US President Donald Trump's transformation of the federal government and recently published a piece describing how she gained hundreds of new sources, leading a colleague to call her "the federal government whisperer".
While classified documents investigations are not unusual, the search of a reporter's home marks an escalation in the government's efforts to crack down on leaks.
Attorney General Pam Bondi said the search was done at the request of the Pentagon.
Trump's administration "will not tolerate illegal leaks of classified information that, when reported, pose a grave risk to our Nation's national security and the brave men and women who are serving our country," Bondi said in a post on X.
An affidavit says the search was related to an investigation into a system administrator in Maryland who authorities allege took home classified reports, the newspaper reported.
The system administrator, Aurelio Perez-Lugones, was charged earlier this month with unlawful retention of national defence information, according to court papers.
Perez-Lugones, who held a top secret security clearance, is accused of printing classified and sensitive reports at work.
In a search of his Maryland home and car this month, authorities found documents marked "SECRET," including one in a lunchbox, according to court papers.
An FBI spokesperson declined to comment on Wednesday.
The Washington Post said on Wednesday that it was monitoring and reviewing the situation.
The Justice Department over the years has developed, and revised, internal guidelines governing how it will respond to news media leaks.
In April, Bondi issued new guidelines saying prosecutors would again have the authority to use subpoenas, court orders and search warrants to hunt for government officials who make "unauthorised disclosures" to journalists.
The moves rescinded a policy of president Joe Biden's administration that protected journalists from having their phone records secretly seized during leak investigations - a practice long decried by news organisations and press freedom groups.
