The Post reported PRISM, founded in 2007, has become the leading source of raw material for the NSA.
Clapper did not directly confirm the program's existence but acknowledged that the Post and Guardian stories "refer to collection of communications pursuant to Section 702 of the Foreign Intelligence Surveillance Act."
That section of the law authorizes intelligence agencies to collect information on non-U.S. residents as part of efforts to gather foreign intelligence.
He said the program cannot be used to target anyone inside the United States or U.S. citizens anywhere in the world and includes "extensive procedures, specifically approved by the court, to ensure that only non-U.S. persons outside the U.S. are targeted, and that minimize the acquisition, retention and dissemination of incidentally acquired information about U.S. persons."
In its stead, the newspaper described a classified report indicating that NSA analysts were allowed to send "content tasking instructions directly to equipment installed at company-controlled locations."
AT&T, Verizon and Comcast declined to comment on the report Friday. Time Warner said it was unfamiliar with PRISM.
Microsoft said Thursday that it doesn't participate in any national security data gathering program. Facebook and Google said they do not give government agencies direct access to their servers.