The website posting is urging Muslim followers to kill the comedian, saying his tongue deserves to be cut.
The contributor, who identified himself as Umar al-Basrawi, was reacting to comments made by Letterman after the U.S. military announced on June 5 that a drone strike in Pakistan had killed al Qaeda leader Ilyas Kashmiri.
Al-Basrawi wrote that Letterman dragged his finger across his neck to show "the way of the slaughter." The threat was posted on the shumukh al-Islam forum, a popular Internet destination for radical Muslims.
The FBI is investigating the threats.
Letterman has been the target of criminal threats in the past. A former CBS News producer was jailed for trying to extort $2 million from Letterman in 2009 by threatening to expose the host's sexual dalliances with members of his staff. A former painter at Letterman's ranch in Montana was jailed following a 2005 plot to kidnap the TV funnyman's nanny and son.
The Associated Press contributed to this report.