Sites including YouTube were banned by the Pentagon in 2007
US troops are to be allowed to use social networking websites such as Twitter and Facebook following a review of internet use and security.
Defence department officials say the benefits of using social media now outweigh the risks to security.
The ruling means that a number of sites blocked by the Pentagon in 2007 - including YouTube - will be unblocked.
However, commanders will be able to temporarily block access to safeguard missions or save bandwidth.
Correspondents say social media and the internet are becoming increasingly important for the US military.
The web is used for recruitment, public relations and for communications between troops and their families, among other things.
US Defence Secretary Robert Gates has said that social networking can help the Pentagon interact with US military employees, many of whom are in their early 20s and regular users of online services.
Chief of tweets
Among the military's higher ranks, Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, has a Twitter feed with more than 16,000 followers.
Joint chiefs chairman Adm Mike Mullen has a large Twitter following
The US Southern Command has also used Twitter for operational updates on relief activities in the aftermath of the Haiti earthquake.
However, the US Marine Corps last year renewed a standing ban on accessing the microblogging service from its network. The US Army also maintained a list of banned social media sites.
But the new policy says visiting websites for pornography, gambling or hate-crime is still banned on military computers.
"We need to take advantage of these capabilities that are out there - this Web 2.0 phenomena," said David Wennergren, deputy assistant secretary of defence for information technology.
"And what we had were inconsistent approaches. Some websites were blocked and some commands were blocking things.
Mr Wennergren said it was more important that serving personnel were trained in how to use online tools effectively and learn about the risks of disclosing information.
"The idea is be responsible and use these tools to help get the job done," he said.
"There are two imperatives. One is the ability to share information. The other is about security - we need to be good at both," he said.
The new rules will only apply to non-classified government computers, the Pentagon said.