Soldiers use the sites to keep in touch with loved ones
A memo from General BB Bell, US Forces Korea commander, says use of YouTube, MySpace and 11 other popular sites via US military portals will be blocked.
The US says the use is taking up too much bandwidth and slows down the military's computer system.
But a US Strategic Command spokesman said a "secondary benefit" was to help operational security.
Personal computers
BBC defence and security correspondent Rob Watson says the decision could stop thousands of soldiers from communicating with friends and loved ones.
For many US soldiers serving overseas, YouTube and other similar websites are a popular way of keeping in touch, he says.
The cyberspace battle space was not one that we were particularly operating well in
Lt Col Christopher Garver, US Army
US takes Iraq war to YouTube
The spokesman for US Strategic Command and Joint Task Force-Global Network Operations said: "As these sites have become more and more popular, they've had an impact on bandwidth resources and network availability and we're having to restrict use of some of them."
He denied that the military was unhappy with the nature of content being posted.
"We're not stopping anybody looking at anything. It's not the nature of the stuff being posted."
However, he added: "Wherever you have sites this popular, you have the potential for malicious activity. A secondary benefit of restricting use of these sites is for operational security."
The block on accessing such websites will not affect those soldiers with their own personal computers. Those, though, are few and far between in places like Iraq and Afghanistan, our correspondent says.
The Pentagon only recently started posting its own videos on YouTube, showing soldiers in action in Iraq in a move designed to reach out to a younger audience and to show the successes of the US military.
In two months, the Multi-National Force-Iraq channel has climbed to 16th in YouTube's most subscribed-to listing and has, the military says, just passed the one million video-views mark.
The BBC's Laura Smith-Spark in Washington says the channel is also a belated attempt to counter the influence of Islamist extremist groups, that have used the internet to post footage of hostages or attacks on US forces.
The Department of Defense seal
The U.S. Department of Defense has announced that access to popular high-bandwidth websites, including user-generated content sites like MySpace and YouTube, will be blocked on military computers. The block takes effect on Monday and will apply worldwide for all military personnel using Department of Defense computers. 

The block is targeted at sites that contain user generated content. Including MySpace and YouTube, a total of 12 sites will be off limits according to Robert Weller of the ''Associated Press'': Metacafe, iFilm, StupidVideos, FileCabi, BlackPlanet, Hi5, Pandora, MTV, 1.fm, live365 and Photobucket.

According to U.S. Strategic Command officials the block is due to "bandwidth/network issues" resulting in possible "clogging of the network". 

The move affects soldiers' ability to view content on these sites, but does not affect the military's ability to provide content. Army Maj. Gen. William Caldwell, the Multinational Force Iraq spokesman, told Carmen L. Gleason of the ''American Forces Press Service'' in mid-March: "We want the American public, from an unfiltered vantage point, to be able to see what coalition forces and Iraqi security forces are doing here in Iraq."