The case was revealed in a court document known as a "criminal information" that the US Justice Department said it filed to stop the statute of limitations expiring earlier in October.
The allegations continue the administration's targeting of Comey, a longtime critic and foe of President Donald Trump who has faced two other criminal cases since the Republican returned to the White House in 2025.
The document filed in Washington federal court charges Comey with transmitting and conspiring to transmit national defence information.
It alleges he shared classified information with a reporter from the New York Times in October 2016.
Comey said in a video on Instagram that "they're back again" and referenced the Justice Department.
"It would almost be funny if it weren't such an appalling abuse of the Department of Justice - one that we cannot become numb to," he said.
"Nothing's changed with me; I'm still innocent, I'm still not afraid of these people - and I still believe in the independent federal judiciary."
Comey's lawyers have already asked a judge to dismiss the case, arguing it is legally improper.
"I think he's a dirty cop," Trump told reporters at Andrews air base when asked about Comey.
"I don't know much about it," he added.
The new case adds to the legal fights facing Comey, whom the Justice Department has already charged twice, accusing him of lying to Congress about media leaks and threatening Trump in a social media post.
A judge dismissed the false statements case, while the threats case remains pending.
Comey's lawyer, Patrick Fitzgerald, said the document "purports to charge Mr Comey with crimes that he did not commit".
The criminal information was filed on October 5, but was unsealed on Monday at the request of Comey's lawyers.
A Justice Department spokesperson described the manoeuvre to halt the statute of limitations as a "widely recognised practice in federal court".
"This is an ongoing criminal investigation, and we will not comment further," the spokesperson said in a statement.
with AP
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