The Site Intelligence Group said a message posted on an extremist website translated as: "Cut the tongue of this lowly Jew and shut it forever".
This is despite the fact that Letterman is not Jewish.
On his late night talk show, the TV host drew his finger across his neck whilst talking about the death of senior al Qaeda figure Ilyas Kashmiri.
Executives at broadcaster CBS and a spokesman for Letterman both declined to comment on the threat.
The FBI is investigating a threat to cut out the tongue of US talk show host David Letterman.
The person who wrote the comment on Tuesday, identified as “Umar al-Basrawi,” instructed users of the Shumuka al-Islam forum to punish Letterman after he made a joke about dead al Qaeda leaders Osama bin Laden and Ilyas Kashmiri on his CBS show June 8.
The fact that David Letterman is Presbyterian, not Jewish, didn’t seem to matter to the poster!
David Letterman has not yet commented publicly on the threats. David Letterman could be called an equal opportunity offender. Now he’s a marked man: a recent posting on the radical Shumukh al-Islam web forum called for his assassination – and, for good measure, to cut off Letterman's tongue. These threats were issued in response to a broadcast the CBS television program ‘Late Show with David Letterman,’ Mr. Letterman celebrated the killing of senior Al Qaeda member Ilyas Kashmiri by drawing his finger across his throat.
Various reports also said that Letterman had insulted Osama bin Laden.
O Allah, amen.”
For the record, Letterman was raised Presbyterian.