Michael Morrell, who served as deputy director and acting director of the CIA, told CBS television's "60 Minutes" program that the former intelligence contractor's disclosures have damaged efforts to track possible terror threats.
"What Edward Snowden did -- has put Americans at greater risk-- because terrorists learn from leaks and they will be more careful, and we will not get the intelligence we would have gotten otherwise," said Morrell, who recently stepped down after 33 years at the CIA.
Snowden has portrayed himself as a whistleblower concerned about National Security Agency eavesdropping and other secret surveillance, but Morrell said the former contractor was a traitor to his country.
"I think this is the most serious leak -- the most serious compromise of classified information in the history of the US intelligence community," he said.
The most damaging disclosure from Snowden exposed the intelligence community's secret budget, or "black book," Morrell said.
Details of what the intelligence agencies spend money on reveal priorities and potential weaknesses that foreign spy services can exploit, he said.
"They could focus their counter-intelligence efforts on those places where we're being successful. And not have to worry as much about those places where we're not being successful," Morrell said, in excerpts of the interview released by CBS.
The full interview will be broadcast Sunday.
Morrell also warned of the dangers posed by political acrimony in Washington after a partial government shutdown dismayed allies and rattled domestic and foreign markets.
"What really keeps me up at night is the inability of our government to make decisions that will push this country forward," he said.
"Any country's national security is more dependent on the strength of its economy and on the strength of its society than anything else."
Morrell served as CIA deputy director from 2010 to 2013, and was named acting director twice, including in November 2012 after retired general David Petraeus resigned after acknowledging an affair with his biographer.