The move comes as the US shifts its military focus from Iraq to Afghanistan, where the number of US troops is expected to reach 68,000 this year from about 32,000 two years ago.
Gates said the expansion was recommended by the army's leadership and strongly backed by Barack Obama, the US president.
This is the second time Gates has increased the size of the army and the Marine Corps since taking office at the end of 2006.
The Pentagon plans to absorb an initial expansion cost of $1.1bn, but Gates has proposed more funding to ensure that "our deploying units are properly manned and not to create new combat formations".
Strains of war
Last week Gates told soldiers at Fort Drum in New York state that a temporary increase in the force's size would enable soldiers to spend longer periods at home between deployments, known in military parlance as "dwell time".
|The boost comes amid the deadliest month for US forces in Afghanistan so far [GALLO/GETTY]
"Dwell time", which is currently 12 months after each 12-month deployment, is seen as vital to maintaining troop morale.
The previous troop expansion Gates ordered in 2006 was recently completed, boosting the US army's size by 65,000 soldiers to 547,000. It also added 27,000 marines to the Marine Corps.
There are currently about 58,000 US soldiers in Afghanistan and 130,000 in Iraq, but between 35,000 and 50,000 troops are expected to pull out of Iraq by August 2010 and a complete withdrawal is planned by the end of 2011.
The plan to boost troop numbers comes as US forces in Afghanistan suffered their deadliest month in the country so far.
Four soldiers were killed in a roadside bomb attack on Monday, bringing July's toll to 27 so far, already surpassing the previous highest total of 26 last month and in September 2008.
It also comes as Private Bowe Bergdahl, a 23-year-old US soldier who had gone missing from his base in eastern Afghanistan, was seen in a video posted on a website by the Taliban over the weekend.
The US military has denounced the release of the video, calling the images Taliban propaganda that violated international law.